RISK FACTORS

Investing in our common stock involves a high degree of risk. You should carefully consider the risks and uncertainties described below, together with all of the other information in this prospectus, including “Management’s Discussion and Analysis of Financial Condition and Results of Operations” and our consolidated financial statements and related notes, before making a decision to invest in our common stock. The risks and uncertainties described below may not be the only ones we face. If any of the risks actually occur, our business, financial condition, operating results and prospects could be materially and adversely affected. In that event, the market price of our common stock could decline, and you could lose part or all of your investment.

Risks Related to Our Business and Our Industry

If we fail to grow our user base, or if user engagement or ad engagement on our platform decline, our revenue, business and operating results may be harmed.

The size of our user base and our users’ level of engagement are critical to our success. We had 231.7 million average MAUs in the three months ended September 30, 2013, which was a 39% increase from 167.1 million average MAUs in the three months ended September 30, 2012. Our financial performance has been and will continue to be significantly determined by our success in growing the number of users and increasing their overall level of engagement on our platform as well as the number of ad engagements. We anticipate that our user growth rate will slow over time as the size of our user base increases. For example, in general, a higher proportion of Internet users in the United States uses Twitter than Internet users in other countries and, in the future, we expect our user growth rate in certain international markets, such as Argentina, France, Japan, Russia, Saudi Arabia and South Africa, to continue to be higher than our user growth rate in the United States. To the extent our user growth rate slows, our success will become increasingly dependent on our ability to increase levels of user engagement and ad engagement on Twitter. We generate a substantial majority of our revenue based upon engagement by our users with the ads that we display. If people do not perceive our products and services to be useful, reliable and trustworthy, we may not be able to attract users or increase the frequency of their engagement with our platform and the ads that we display. A number of consumer-oriented websites that achieved early popularity have since seen their user bases or levels of engagement decline, in some cases precipitously. There is no guarantee that we will not experience a similar erosion of our user base or engagement levels. A number of factors could potentially negatively affect user growth and engagement, including if:

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If we are unable to increase our user growth or engagement, or if they decline, this could result in our products and services being less attractive to potential new users, as well as advertisers, which would have a material and adverse impact on our business, financial condition and operating results.

If our users do not continue to contribute content or their contributions are not valuable to other users, we may experience a decline in the number of users accessing our products and services and user engagement, which could result in the loss of advertisers and revenue.

Our success depends on our ability to provide users of our products and services with valuable content, which in turn depends on the content contributed by our users. We believe that one of our competitive advantages is the quality, quantity and real-time nature of the content on Twitter, and that access to unique or real-time content is one of the main reasons users visit Twitter. Our ability to expand into new international markets depends on the availability of relevant local content in those markets. We seek to foster a broad and engaged user community, and we encourage world leaders, government officials, celebrities, athletes, journalists, sports teams, media outlets and brands to use our products and services to express their views to broad audiences. We also encourage media outlets to use our products and services to distribute their content. If users, including influential users, do not continue to contribute content to Twitter, and we are unable to provide users with valuable and timely content, our user base and user engagement may decline. Additionally, if we are not able to address user concerns regarding the safety and security of our products and services or if we are unable to successfully prevent abusive or other hostile behavior on our platform, the size of our user base and user engagement may decline. We rely on the sale of advertising services for the substantial majority of our revenue. If we experience a decline in the number of users or a decline in user engagement, including as a result of the loss of world leaders, government officials, celebrities, athletes, journalists, sports teams, media outlets and brands who generate content on Twitter, advertisers may not view our products and services as attractive for their marketing expenditures, and may reduce their spending with us which would harm our business and operating results.

We generate the substantial majority of our revenue from advertising. The loss of advertising revenue could harm our business.

The substantial majority of our revenue is currently generated from third parties advertising on Twitter. We generated 85% and 89% of our revenue from advertising in 2012 and the nine months ended September 30, 2013, respectively. We generate substantially all of our advertising revenue through the sale of our three Promoted Products: Promoted Tweets, Promoted Accounts and Promoted Trends. As is common in the industry, our advertisers do not have long-term advertising commitments with us. In addition, many of our advertisers purchase our advertising services through one of several large advertising agency holding companies. Advertising agencies and potential new advertisers may view our Promoted Products as experimental and unproven, and we may need to devote additional time and resources to educate them about our products and services. Advertisers

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also may choose to reach users through our free products and services, instead of our Promoted Products. Advertisers will not continue to do business with us, or they will reduce the prices they are willing to pay to advertise with us, if we do not deliver ads in an effective manner, or if they do not believe that their investment in advertising with us will generate a competitive return relative to alternatives, including online, mobile and traditional advertising platforms. Our advertising revenue could be adversely affected by a number of other factors, including:

The occurrence of any of these or other factors could result in a reduction in demand for our ads, which may reduce the prices we receive for our ads, either of which would negatively affect our revenue and operating results.

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If we are unable to compete effectively for users and advertiser spend, our business and operating results could be harmed.

Competition for users of our products and services is intense. Although we have developed a new global platform for public self-expression and conversation in real time, we face strong competition in our business. We compete against many companies to attract and engage users, including companies which have greater financial resources and substantially larger user bases, such as Facebook (including Instagram), Google, LinkedIn, Microsoft and Yahoo!, which offer a variety of Internet and mobile device-based products, services and content. For example, Facebook operates a social networking site with significantly more users than Twitter and has been introducing features similar to those of Twitter. In addition, Google may use its strong position in one or more markets to gain a competitive advantage over us in areas in which we operate, including by integrating competing features into products or services they control. As a result, our competitors may acquire and engage users at the expense of the growth or engagement of our user base, which would negatively affect our business. We also compete against smaller companies, such as Sina Weibo, LINE and Kakao, each of which is based in Asia.

We believe that our ability to compete effectively for users depends upon many factors both within and beyond our control, including:

We also face significant competition for advertiser spend. The substantial majority of our revenue is currently generated through ads on Twitter, and we compete against online and mobile businesses, including those referenced above, and traditional media outlets, such as television, radio and print, for advertising budgets. In order to grow our revenue and improve our operating results, we must increase our share of spending on advertising relative to our competitors, many of which are larger companies that offer more traditional and widely accepted advertising products. In addition, some of our larger competitors have substantially broader product or service offerings and leverage their relationships based on other products or services to gain additional share of advertising budgets.

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We believe that our ability to compete effectively for advertiser spend depends upon many factors both within and beyond our control, including:

In recent years, there have been significant acquisitions and consolidation by and among our actual and potential competitors. We anticipate this trend of consolidation will continue, which will present heightened competitive challenges for our business. Acquisitions by our competitors may result in reduced functionality of our products and services. For example, following Facebook’s acquisition of Instagram, Facebook disabled Instagram’s photo integration with Twitter such that Instagram photos are no longer viewable within Tweets and users are now re-directed to Instagram to view Instagram photos through a link within a Tweet. As a result, our users may be less likely to click on links to Instagram photos in Tweets, and Instagram users may be less likely to tweet or remain active users of Twitter. Any similar elimination of integration with Twitter in the future, whether by Facebook or others, may adversely impact our business and operating results.

Consolidation may also enable our larger competitors to offer bundled or integrated products that feature alternatives to our platform. Reduced functionality of our products and services, or our competitors’ ability to offer bundled or integrated products that compete directly with us, may cause our user growth, user engagement and ad engagement to decline and advertisers to reduce their spend with us.

If we are not able to compete effectively for users and advertiser spend our business and operating results would be materially and adversely affected.

Our operating results may fluctuate from quarter to quarter, which makes them difficult to predict.

Our quarterly operating results have fluctuated in the past and will fluctuate in the future. As a result, our past quarterly operating results are not necessarily indicators of future performance. Our operating results in any given quarter can be influenced by numerous factors, many of which we are unable to predict or are outside of our control, including:

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Given our limited operating history and the rapidly evolving markets in which we compete, our historical operating results may not be useful to you in predicting our future operating results. We believe our rapid growth may understate the potential seasonality of our business. As our revenue growth rate slows, we expect that the seasonality in our business may become more pronounced and may in the future cause our operating results to fluctuate. For example, advertising spending is traditionally seasonally strong in the fourth quarter of each year and we believe that this seasonality affects our quarterly results, which generally reflect higher sequential advertising revenue growth from the third to fourth quarter compared to sequential advertising revenue growth from the fourth quarter to the subsequent first quarter. In addition, global economic concerns continue to create uncertainty and unpredictability and add risk to our future outlook. An economic downturn in any particular region in which we do business or globally could result in reductions in advertising revenue, as our advertisers reduce their advertising budgets, and other adverse effects that could harm our operating results.

User growth and engagement depend upon effective interoperation with operating systems, networks, devices, web browsers and standards that we do not control.

We make our products and services available across a variety of operating systems and through websites. We are dependent on the interoperability of our products and services with popular devices, desktop and mobile operating systems and web browsers that we do not control, such as Mac OS, Windows, Android, iOS, Chrome and Firefox. Any changes in such systems, devices or web browsers that degrade the functionality of our products and services or give preferential treatment to competitive products or services could adversely affect usage of our products and services. Further, if the number of platforms for which we develop our product expands, it will result in an increase in our operating expenses. In order to deliver high quality products and services, it is important that our products and

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services work well with a range of operating systems, networks, devices, web browsers and standards that we do not control. In addition, because a majority of our users access our products and services through mobile devices, we are particularly dependent on the interoperability of our products and services with mobile devices and operating systems. We may not be successful in developing relationships with key participants in the mobile industry or in developing products or services that operate effectively with these operating systems, networks, devices, web browsers and standards. In the event that it is difficult for our users to access and use our products and services, particularly on their mobile devices, our user growth and engagement could be harmed, and our business and operating results could be adversely affected.

If we fail to expand effectively in international markets, our revenue and our business will be harmed.

We may not be able to monetize our products and services internationally as effectively as in the United States as a result of competition, advertiser demand, differences in the digital advertising market and digital advertising conventions, as well as differences in the way that users in different countries access or utilize our products and services. Differences in the competitive landscape in international markets may impact our ability to monetize our products and services. For example, in South Korea we face intense competition from a messaging service offered by Kakao, which offers some of the same communication features as Twitter. The existence of a well-established competitor in an international market may adversely affect our ability to increase our user base, attract advertisers and monetize our products in such market. We may also experience differences in advertiser demand in international markets. For example, during times of political upheaval, advertisers may choose not to advertise on Twitter. Certain international markets are also not as familiar with digital advertising in general, or in new forms of digital advertising such as our Promoted Products. Further, we face challenges in providing certain advertising products, features or analytics in certain international markets, such as the European Union, due to government regulation. Our products and services may also be used differently abroad than in the United States. In particular, in certain international markets where Internet access is not as rapid or reliable as in the United States, users tend not to take advantage of certain features of our products and services, such as rich media included in Tweets. Additionally, in certain emerging markets, such as India, many users access our products and services through feature phones with limited functionality, rather than through smartphones, our website or desktop applications. This limits our ability to deliver certain features to those users and may limit the ability of advertisers to deliver compelling advertisements to users in these markets which may result in reduced ad engagements which would adversely affect our business and operating results.

If our revenue from our international operations, and particularly from operations in the countries and regions on which we have focused our spending, does not exceed the expense of establishing and maintaining these operations, our business and operating results will suffer. In addition, our user base may expand more rapidly in international regions where we are less successful in monetizing our products and services. As our user base continues to expand internationally, we will need to increase revenue from the activity generated by our international users in order to grow our business. For example, users outside the United States constituted 77% of our average MAUs in the three months ended September 30, 2013, but our international revenue, as determined based on the billing location of our advertisers, was only 26% of our consolidated revenue in the three months ended September 30, 2013. Our inability to successfully expand internationally could adversely affect our business, financial condition and operating results.

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We have a limited operating history in a new and unproven market for our platform, which makes it difficult to evaluate our future prospects and may increase the risk that we will not be successful.

We have developed a global platform for public self-expression and conversation in real time, and the market for our products and services is relatively new and may not develop as expected, if at all. People who are not our users may not understand the value of our products and services and new users may initially find our product confusing. There may be a perception that our products and services are only useful to users who tweet, or to influential users with large audiences. Convincing potential new users of the value of our products and services is critical to increasing our user base and to the success of our business.

We have a limited operating history, and only began to generate revenue in 2009 and we started to sell our Promoted Products in 2010, which makes it difficult to effectively assess our future prospects or forecast our future results. You should consider our business and prospects in light of the risks and challenges we encounter or may encounter in this developing and rapidly evolving market. These risks and challenges include our ability to, among other things:

If we fail to educate potential users and potential advertisers about the value of our products and services, if the market for our platform does not develop as we expect or if we fail to address the needs of this market, our business will be harmed. We may not be able to successfully address these risks and challenges or others. Failure to adequately address these risks and challenges could harm our business and cause our operating results to suffer.

We have incurred significant operating losses in the past, and we may not be able to achieve or subsequently maintain profitability.

Since our inception, we have incurred significant operating losses, and, as of September 30, 2013, we had an accumulated deficit of $483.2 million. Although our revenue has grown rapidly, increasing from $28.3 million in 2010 to $316.9 million in 2012, we expect that our revenue growth rate

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will slow in the future as a result of a variety of factors, including the gradual slow down in the growth rate of our user base. We believe that our future revenue growth will depend on, among other factors, our ability to attract new users, increase user engagement and ad engagement, increase our brand awareness, compete effectively, maximize our sales efforts, demonstrate a positive return on investment for advertisers, successfully develop new products and services and expand internationally. Accordingly, you should not rely on the revenue growth of any prior quarterly or annual period as an indication of our future performance. We also expect our costs to increase in future periods as we continue to expend substantial financial resources on:

These investments may not result in increased revenue or growth in our business.

In addition, we have granted stock options and RSUs to our employees. RSUs granted to domestic employees before February 2013 and all RSUs granted to international employees, or the Pre-2013 RSUs, vest upon the satisfaction of both a service condition and a performance condition. The service condition for a majority of the Pre-2013 RSUs is satisfied over a period of four years. The performance condition will be satisfied on the earlier of (i) the date that is the earlier of (x) six months after the effective date of this offering or (y) March 8th of the calendar year following the effective date of this offering (which we may elect to accelerate to February 15th); and (ii) the date of a change in control. As of September 30, 2013, no stock-based compensation expense had been recognized for the Pre-2013 RSUs because a qualifying event as described above was not probable. In the quarter in which this offering is completed, we will begin recording stock-based compensation expense based on the grant-date fair value of the Pre-2013 RSUs using the accelerated attribution method, net of estimated forfeitures. If this offering had been completed on September 30, 2013, we would have recorded $385.2 million of cumulative stock-based compensation expense related to the Pre-2013 RSUs on that date, and an additional $199.6 million of unrecognized stock-based compensation expense related to the Pre-2013 RSUs, net of estimated forfeitures, would be recognized over a weighted-average period of approximately three years. In addition to stock-based compensation expense associated with the Pre-2013 RSUs, as of September 30, 2013, we had unrecognized stock-based compensation expense of approximately $698.3 million related to other outstanding equity awards, after giving effect to estimated forfeitures, which we expect to recognize over a weighted-average period of approximately four years. Following the completion of this offering, the stock-based compensation expense related to Pre-2013 RSUs and other outstanding equity awards will have a significant negative impact on our ability to achieve profitability on a GAAP basis in 2013 and 2014.

If we are unable to generate adequate revenue growth and to manage our expenses, we may continue to incur significant losses in the future and may not be able to achieve or maintain profitability.

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Our business depends on continued and unimpeded access to our products and services on the Internet by our users and advertisers. If we or our users experience disruptions in Internet service or if Internet service providers are able to block, degrade or charge for access to our products and services, we could incur additional expenses and the loss of users and advertisers.

We depend on the ability of our users and advertisers to access the Internet. Currently, this access is provided by companies that have significant market power in the broadband and Internet access marketplace, including incumbent telephone companies, cable companies, mobile communications companies, government-owned service providers, device manufacturers and operating system providers, any of whom could take actions that degrade, disrupt or increase the cost of user access to our products or services, which would, in turn, negatively impact our business. For example, access to Twitter is blocked in China. The adoption of any laws or regulations that adversely affect the growth, popularity or use of the Internet, including laws or practices limiting Internet neutrality, could decrease the demand for, or the usage of, our products and services, increase our cost of doing business and adversely affect our operating results. We also rely on other companies to maintain reliable network systems that provide adequate speed, data capacity and security to us and our users. As the Internet continues to experience growth in the number of users, frequency of use and amount of data transmitted, the Internet infrastructure that we and our users rely on may be unable to support the demands placed upon it. The failure of the Internet infrastructure that we or our users rely on, even for a short period of time, could undermine our operations and harm our operating results.

Our new products, services and initiatives and changes to existing products, services and initiatives could fail to attract users and advertisers or generate revenue.

Our ability to increase the size and engagement of our user base, attract advertisers and generate revenue will depend in part on our ability to create successful new products and services, both independently and in conjunction with third parties. We may introduce significant changes to our existing products and services or develop and introduce new and unproven products and services, including technologies with which we have little or no prior development or operating experience. For example, in 2013, we introduced Vine, a mobile application that enables users to create and distribute videos that are up to six seconds in length, and #Music, a mobile application that helps users discover new music and artists based on Twitter data. If new or enhanced products or services fail to engage users and advertisers, we may fail to attract or retain users or to generate sufficient revenue or operating profit to justify our investments, and our business and operating results could be adversely affected. In addition, we have launched and expect to continue to launch strategic initiatives, such as the Nielsen Twitter TV Rating, that do not directly generate revenue but which we believe will enhance our attractiveness to users and advertisers. In the future, we may invest in new products, services and initiatives to generate revenue, but there is no guarantee these approaches will be successful. We may not be successful in future efforts to generate revenue from our new products or services. If our strategic initiatives do not enhance our ability to monetize our existing products and services or enable us to develop new approaches to monetization, we may not be able to maintain or grow our revenue or recover any associated development costs and our operating results could be adversely affected.

Spam could diminish the user experience on our platform, which could damage our reputation and deter our current and potential users from using our products and services.

“Spam” on Twitter refers to a range of abusive activities that are prohibited by our terms of service and is generally defined as unsolicited, repeated actions that negatively impact other users with the general goal of drawing user attention to a given account, site, product or idea. This includes posting large numbers of unsolicited mentions of a user, duplicate Tweets, misleading links (e.g., to malware or click-jacking pages) or other false or misleading content, and aggressively following and un-following accounts, adding users to lists, sending invitations, retweeting and favoriting Tweets to

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inappropriately attract attention. Our terms of service also prohibit the creation of serial or bulk accounts, both manually or using automation, for disruptive or abusive purposes, such as to tweet spam or to artificially inflate the popularity of users seeking to promote themselves on Twitter. Although we continue to invest resources to reduce spam on Twitter, we expect spammers will continue to seek ways to act inappropriately on our platform. In addition, we expect that increases in the number of users on our platform will result in increased efforts by spammers to misuse our platform. We continuously combat spam, including by suspending or terminating accounts we believe to be spammers and launching algorithmic changes focused on curbing abusive activities. Our actions to combat spam require the diversion of significant time and focus of our engineering team from improving our products and services. If spam increases on Twitter, this could hurt our reputation for delivering relevant content or reduce user growth and user engagement and result in continuing operational cost to us.

If we fail to effectively manage our growth, our business and operating results could be harmed.

We continue to experience rapid growth in our headcount and operations, which will continue to place significant demands on our management, operational and financial infrastructure. As of September 30, 2013, we had over 2,300 employees, an increase of over 2,200 employees since January 1, 2010. We intend to continue to make substantial investments to expand our operations, research and development, sales and marketing and general and administrative organizations, as well as our international operations. We face significant competition for employees, particularly engineers, designers and product managers, from other Internet and high-growth companies, which include both publicly-traded and privately-held companies, and we may not be able to hire new employees quickly enough to meet our needs. To attract highly skilled personnel, we have had to offer, and believe we will need to continue to offer, highly competitive compensation packages. In addition, as we have grown, we have significantly expanded our operating lease commitments. As we continue to grow, we are subject to the risks of over-hiring, over-compensating our employees and over-expanding our operating infrastructure, and to the challenges of integrating, developing and motivating a rapidly growing employee base in various countries around the world. In addition, we may not be able to innovate or execute as quickly as a smaller, more efficient organization. If we fail to effectively manage our hiring needs and successfully integrate our new hires, our efficiency and ability to meet our forecasts and our employee morale, productivity and retention could suffer, and our business and operating results could be adversely affected.

Providing our products and services to our users is costly and we expect our expenses to continue to increase in the future as we broaden our user base and increase user engagement, as users increase the amount of content they contribute, and as we develop and implement new features, products and services that require more infrastructure, such as our mobile video product, Vine. In addition, our operating expenses, such as our research and development expenses and sales and marketing expenses, have grown rapidly as we have expanded our business. Historically, our costs have increased each year due to these factors and we expect to continue to incur increasing costs to support our anticipated future growth. We expect to continue to invest in our infrastructure in order to enable us to provide our products and services rapidly and reliably to users around the world, including in countries where we do not expect significant near-term monetization. Continued growth could also strain our ability to maintain reliable service levels for our users and advertisers, develop and improve our operational, financial, legal and management controls, and enhance our reporting systems and procedures. As a public company we will incur significant legal, accounting and other expenses that we did not incur as a private company. Our expenses may grow faster than our revenue, and our expenses may be greater than we anticipate. Managing our growth will require significant expenditures and allocation of valuable management resources. If we fail to achieve the necessary level of efficiency in our organization as it grows, our business, operating results and financial condition would be harmed.

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Our business and operating results may be harmed by a disruption in our service, or by our failure to timely and effectively scale and adapt our existing technology and infrastructure.

One of the reasons people come to Twitter is for real-time information. We have experienced, and may in the future experience, service disruptions, outages and other performance problems due to a variety of factors, including infrastructure changes, human or software errors, hardware failure, capacity constraints due to an overwhelming number of people accessing our products and services simultaneously, computer viruses and denial of service or fraud or security attacks. Although we are investing significantly to improve the capacity, capability and reliability of our infrastructure, we are not currently serving traffic equally through our co-located data centers that support our platform. Accordingly, in the event of a significant issue at the data center supporting most of our network traffic, some of our products and services may become inaccessible to the public or the public may experience difficulties accessing our products and services. For example, in July 2012, due to the failure of two parallel systems at nearly the same time in one of our data centers, Twitter became inaccessible for approximately two hours. Any disruption or failure in our infrastructure could hinder our ability to handle existing or increased traffic on our platform, which could significantly harm our business.

As the number of our users increases and our users generate more content, including photos and videos hosted by Twitter, we may be required to expand and adapt our technology and infrastructure to continue to reliably store, serve and analyze this content. It may become increasingly difficult to maintain and improve the performance of our products and services, especially during peak usage times, as our products and services become more complex and our user traffic increases. In addition, because we lease our data center facilities, we cannot be assured that we will be able to expand our data center infrastructure to meet user demand in a timely manner, or on favorable economic terms. If our users are unable to access Twitter or we are not able to make information available rapidly on Twitter, users may seek other channels to obtain the information, and may not return to Twitter or use Twitter as often in the future, or at all. This would negatively impact our ability to attract users and advertisers and increase engagement of our users. We expect to continue to make significant investments to maintain and improve the capacity, capability and reliability of our infrastructure. To the extent that we do not effectively address capacity constraints, upgrade our systems as needed and continually develop our technology and infrastructure to accommodate actual and anticipated changes in technology, our business and operating results may be harmed.

Action by governments to restrict access to our products and services or censor Twitter content could harm our business and operating results.

Governments have sought, and may in the future seek, to censor content available through our products and services, restrict access to our products and services from their country entirely or impose other restrictions that may affect the accessibility of our products and services for an extended period of time or indefinitely. For example, domestic Internet service providers in China have blocked access to Twitter, and other countries, including Iran, Libya, Pakistan and Syria, have intermittently restricted access to Twitter, and we believe that access to Twitter has been blocked in these countries primarily for political reasons. In addition, governments in other countries may seek to restrict access to our products and services if they consider us to be in violation of their laws. In the event that access to our products and services is restricted, in whole or in part, in one or more countries or our competitors are able to successfully penetrate geographic markets that we cannot access, our ability to retain or increase our user base and user engagement may be adversely affected, and our operating results may be harmed.

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If we are unable to maintain and promote our brand, our business and operating results may be harmed.

We believe that maintaining and promoting our brand is critical to expanding our base of users and advertisers. Maintaining and promoting our brand will depend largely on our ability to continue to provide useful, reliable and innovative products and services, which we may not do successfully. We may introduce new features, products, services or terms of service that users, platform partners or advertisers do not like, which may negatively affect our brand. Additionally, the actions of platform partners may affect our brand if users do not have a positive experience using third-party applications or websites integrated with Twitter or that make use of Twitter content. Our brand may also be negatively affected by the actions of users that are hostile or inappropriate to other people, by users impersonating other people, by users identified as spam, by users introducing excessive amounts of spam on our platform or by third parties obtaining control over users’ accounts. For example, in April 2013, attackers obtained the credentials to the Twitter account of the Associated Press news service through a “phishing” attack targeting Associated Press employees. The attackers posted an erroneous Tweet from the Associated Press account reporting that there had been explosions at the White House, triggering a stock market decline, and focusing media attention on our brand and security efforts. Maintaining and enhancing our brand may require us to make substantial investments and these investments may not achieve the desired goals. If we fail to successfully promote and maintain our brand or if we incur excessive expenses in this effort, our business and operating results could be adversely affected.

Negative publicity could adversely affect our business and operating results.

We receive a high degree of media coverage around the world. Negative publicity about our company, including about our product quality and reliability, changes to our products and services, privacy and security practices, litigation, regulatory activity, the actions of our users or user experience with our products and services, even if inaccurate, could adversely affect our reputation and the confidence in and the use of our products and services. For example, service outages on Twitter typically result in widespread media reports. Such negative publicity could also have an adverse effect on the size, engagement and loyalty of our user base and result in decreased revenue, which could adversely affect our business and operating results.

Our future performance depends in part on support from platform partners and data partners.

We believe user engagement with our products and services depends in part on the availability of applications and content generated by platform partners. Beginning in 2012, we launched Twitter Cards, which allow platform partners to ensure that whenever they or any user tweets from their websites or applications, the Tweet will automatically include rich content like a photo, a video, a sound clip, an article summary or information about a product, and make it instantly accessible to any other user on Twitter. Twitter Cards allow platform partners to create lightweight interactive applications to promote their content or their products. The availability and development of these applications and content depends on platform partners’ perceptions and analysis of the relative benefits of developing applications and content for our products and services. If platform partners focus their efforts on other platforms, the availability and quality of applications and content for our products and services may suffer. There is no assurance that platform partners will continue to develop and maintain applications and content for our products and services. If platform partners cease to develop and maintain applications and content for our products and services, user engagement may decline. In addition, we generate revenue from licensing our historical and real-time data to third parties. If any of these relationships are terminated or not renewed, or if we are unable to enter into similar relationships in the future, our operating results could be adversely affected.

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We focus on product innovation and user engagement rather than short-term operating results.

We encourage employees to quickly develop and help us launch new and innovative features. We focus on improving the user experience for our products and services and on developing new and improved products and services for the advertisers on our platform. We prioritize innovation and the experience for users and advertisers on our platform over short-term operating results. We frequently make product and service decisions that may reduce our short-term operating results if we believe that the decisions are consistent with our goals to improve the user experience and performance for advertisers, which we believe will improve our operating results over the long term. These decisions may not be consistent with the short-term expectations of investors and may not produce the long-term benefits that we expect, in which case our user growth and user engagement, our relationships with advertisers and our business and operating results could be harmed. In addition, our focus on the user experience may negatively impact our relationships with our existing or prospective advertisers. This could result in a loss of advertisers, which could harm our revenue and operating results.

Our international operations are subject to increased challenges and risks.

We have offices around the world and our products and services are available in multiple languages. We expect to continue to expand our international operations in the future by opening offices in new jurisdictions and expanding our offerings in new languages. However, we have limited operating history outside the United States, and our ability to manage our business and conduct our operations internationally requires considerable management attention and resources and is subject to the particular challenges of supporting a rapidly growing business in an environment of multiple languages, cultures, customs, legal and regulatory systems, alternative dispute systems and commercial markets. International expansion has required and will continue to require us to invest significant funds and other resources. Operating internationally subjects us to new risks and may increase risks that we currently face, including risks associated with:

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If we are unable to manage the complexity of our global operations successfully, our business, financial condition and operating results could be adversely affected.

Our products and services may contain undetected software errors, which could harm our business and operating results.

Our products and services incorporate complex software and we encourage employees to quickly develop and help us launch new and innovative features. Our software has contained, and may now or in the future contain, errors, bugs or vulnerabilities. For example, we experienced a service outage in June 2012 during which Twitter service was inaccessible for approximately two hours as a result of a cascading software bug in one of our infrastructure components. Some errors in our software code may only be discovered after the product or service has been released. Any errors, bugs or vulnerabilities discovered in our code after release could result in damage to our reputation, loss of users, loss of platform partners, loss of advertisers or advertising revenue or liability for damages, any of which could adversely affect our business and operating results.

Our business is subject to complex and evolving U.S. and foreign laws and regulations. These laws and regulations are subject to change and uncertain interpretation, and could result in claims, changes to our business practices, monetary penalties, increased cost of operations or declines in user growth, user engagement or ad engagement, or otherwise harm our business.

We are subject to a variety of laws and regulations in the United States and abroad that involve matters central to our business, including privacy, rights of publicity, data protection, content regulation, intellectual property, competition, protection of minors, consumer protection and taxation. Many of these laws and regulations are still evolving and being tested in courts and could be interpreted or applied in ways that could harm our business, particularly in the new and rapidly evolving industry in which we operate. The introduction of new products or services may subject us to additional laws and regulations. In addition, foreign data protection, privacy, consumer protection, content regulation and other laws and regulations are often more restrictive than those in the United States. In particular, the European Union and its member states traditionally have taken broader views as to types of data that are subject to privacy and data protection, and have imposed greater legal obligations on companies in this regard. A number of proposals are pending before federal, state and foreign legislative and regulatory bodies that could significantly affect our business. For example, regulation relating to the 1995 European Union Data Protection Directive is currently being considered by European legislative bodies that may include more stringent operational requirements for entities processing personal information and significant penalties for non-compliance. Similarly, there have been a number of recent

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legislative proposals in the United States, at both the federal and state level, that would impose new obligations in areas such as privacy and liability for copyright infringement by third parties. The U.S. government, including the Federal Trade Commission, or the FTC, and the Department of Commerce, has announced that it is reviewing the need for greater regulation for the collection of information concerning user behavior on the Internet, including regulation aimed at restricting certain online tracking and targeted advertising practices. Additionally, recent amendments to U.S. patent laws may affect the ability of companies, including us, to protect their innovations and defend against claims of patent infringement. We currently allow use of our platform without the collection of extensive personal information, such as age. We may experience additional pressure to expand our collection of personal information in order to comply with new and additional regulatory demands or we may independently decide to do so. Having additional personal information may subject us to additional regulation. Further, it is difficult to predict how existing laws and regulations will be applied to our business and the new laws and regulations to which we may become subject, and it is possible that they may be interpreted and applied in a manner that is inconsistent with our practices. These existing and proposed laws and regulations can be costly to comply with and can delay or impede the development of new products and services, result in negative publicity, significantly increase our operating costs, require significant time and attention of management and technical personnel and subject us to inquiries or investigations, claims or other remedies, including fines or demands that we modify or cease existing business practices.

Regulatory investigations and settlements could cause us to incur additional expenses or change our business practices in a manner materially adverse to our business.

We have been subject to regulatory investigations in the past, and expect to continue to be subject to regulatory scrutiny as our business grows and awareness of our brand increases. In March 2011, to resolve an investigation into various incidents, we entered into a settlement agreement with the FTC that, among other things, requires us to establish an information security program designed to protect non-public consumer information and also requires that we obtain biennial independent security assessments. The obligations under the settlement agreement remain in effect until the latter of March 2, 2031, or the date 20 years after the date, if any, on which the U.S. government or the FTC files a complaint in federal court alleging any violation of the order. We expect to continue to be the subject of regulatory inquiries, investigations and audits in the future by the FTC and other regulators around the world.

It is possible that a regulatory inquiry, investigation or audit might result in changes to our policies or practices, and may cause us to incur substantial costs or could result in reputational harm, prevent us from offering certain products, services, features or functionalities, cause us to incur substantial costs or require us to change our business practices in a manner materially adverse to our business. Violation of existing or future regulatory orders, settlements or consent decrees could subject us to substantial monetary fines and other penalties that could negatively affect our financial condition and operating results.

Even though Twitter is a global platform for public self-expression and conversation, user trust regarding privacy is important to the growth of users and the increase in user engagement on our platform, and privacy concerns relating to our products and services could damage our reputation and deter current and potential users and advertisers from using Twitter.

From time to time, concerns have been expressed by governments, regulators and others about whether our products, services or practices compromise the privacy of users and others. Concerns about, governmental or regulatory actions involving our practices with regard to the collection, use, disclosure or security of personal information or other privacy-related matters, even if unfounded, could damage our reputation, cause us to lose users and advertisers and adversely affect our operating results. While we strive to comply with applicable data protection laws and regulations, as well as our

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own posted privacy policies and other obligations we may have with respect to privacy and data protection, the failure or perceived failure to comply may result, and in some cases has resulted, in inquiries and other proceedings or actions against us by governments, regulators or others, as well as negative publicity and damage to our reputation and brand, each of which could cause us to lose users and advertisers, which could have an adverse effect on our business.

Any systems failure or compromise of our security that results in the unauthorized access to or release of our users’ or advertisers’ data could significantly limit the adoption of our products and services, as well as harm our reputation and brand and, therefore, our business. We expect to continue to expend significant resources to protect against security breaches. The risk that these types of events could seriously harm our business is likely to increase as we expand the number of products and services we offer, increase the size of our user base and operate in more countries.

Governments and regulators around the world are considering a number of legislative and regulatory proposals concerning data protection. In addition, the interpretation and application of consumer and data protection laws or regulations in the United States, Europe and elsewhere are often uncertain and in flux, and in some cases, laws or regulations in one country may be inconsistent with, or contrary to, those of another country. It is possible that these laws and regulations may be interpreted and applied in a manner that is inconsistent with our practices. If so, in addition to the possibility of fines, this could result in an order requiring that we change our practices, which could have an adverse effect on our business and operating results. Complying with new laws and regulations could cause us to incur substantial costs or require us to change our business practices in a manner materially adverse to our business.

If our security measures are breached, or if our products and services are subject to attacks that degrade or deny the ability of users to access our products and services, our products and services may be perceived as not being secure, users and advertisers may curtail or stop using our products and services and our business and operating results could be harmed.

Our products and services involve the storage and transmission of users’ and advertisers’ information, and security breaches expose us to a risk of loss of this information, litigation and potential liability. We experience cyber-attacks of varying degrees on a regular basis, and as a result, unauthorized parties have obtained, and may in the future obtain, access to our data or our users’ or advertisers’ data. For example, in February 2013, we disclosed that sophisticated unknown third parties had attacked our systems and may have had access to limited information for approximately 250,000 users. Our security measures may also be breached due to employee error, malfeasance or otherwise. Additionally, outside parties may attempt to fraudulently induce employees, users or advertisers to disclose sensitive information in order to gain access to our data or our users’ or advertisers’ data or accounts, or may otherwise obtain access to such data or accounts. Since our users and advertisers may use their Twitter accounts to establish and maintain online identities, unauthorized communications from Twitter accounts that have been compromised may damage their reputations and brands as well as ours. Any such breach or unauthorized access could result in significant legal and financial exposure, damage to our reputation and a loss of confidence in the security of our products and services that could have an adverse effect on our business and operating results. Because the techniques used to obtain unauthorized access, disable or degrade service or sabotage systems change frequently and often are not recognized until launched against a target, we may be unable to anticipate these techniques or to implement adequate preventative measures. If an actual or perceived breach of our security occurs, the market perception of the effectiveness of our security measures could be harmed, we could lose users and advertisers and we may incur significant legal and financial exposure, including legal claims and regulatory fines and penalties. Any of these actions could have a material and adverse effect on our business, reputation and operating results.

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We may face lawsuits or incur liability as a result of content published or made available through our products and services.

We have faced and will continue to face claims relating to content that is published or made available through our products and services or third party products or services. In particular, the nature of our business exposes us to claims related to defamation, intellectual property rights, rights of publicity and privacy, illegal content, content regulation and personal injury torts. The law relating to the liability of providers of online products or services for activities of their users remains somewhat unsettled, both within the United States and internationally. This risk may be enhanced in certain jurisdictions outside the United States where we may be less protected under local laws than we are in the United States. In addition, the public nature of communications on our network exposes us to risks arising from the creation of impersonation accounts intended to be attributed to our users or advertisers. We could incur significant costs investigating and defending these claims. If we incur costs or liability as a result of these events occurring, our business, financial condition and operating results could be adversely affected.

Our intellectual property rights are valuable, and any inability to protect them could reduce the value of our products, services and brand.

Our trade secrets, trademarks, copyrights, patents and other intellectual property rights are important assets for us. We rely on, and expect to continue to rely on, a combination of confidentiality and license agreements with our employees, consultants and third parties with whom we have relationships, as well as trademark, trade dress, domain name, copyright, trade secret and patent laws, to protect our brand and other intellectual property rights. However, various events outside of our control pose a threat to our intellectual property rights, as well as to our products, services and technologies. For example, we may fail to obtain effective intellectual property protection, or effective intellectual property protection may not be available in every country in which our products and services are available. Also, the efforts we have taken to protect our intellectual property rights may not be sufficient or effective, and any of our intellectual property rights may be challenged, which could result in them being narrowed in scope or declared invalid or unenforceable. There can be no assurance our intellectual property rights will be sufficient to protect against others offering products or services that are substantially similar to ours and compete with our business.

We rely on non-patented proprietary information and technology, such as trade secrets, confidential information, know-how and technical information. While in certain cases we have agreements in place with employees and third parties that place restrictions on the use and disclosure of this intellectual property, these agreements may be breached, or this intellectual property may otherwise be disclosed or become known to our competitors, which could cause us to lose any competitive advantage resulting from this intellectual property.

We are pursuing registration of trademarks and domain names in the United States and in certain jurisdictions outside of the United States. Effective protection of trademarks and domain names is expensive and difficult to maintain, both in terms of application and registration costs as well as the costs of defending and enforcing those rights. We may be required to protect our rights in an increasing number of countries, a process that is expensive and may not be successful or which we may not pursue in every country in which our products and services are distributed or made available.

We are party to numerous agreements that grant licenses to third parties to use our intellectual property, including our trademarks. For example, many third parties distribute their content through Twitter, or embed Twitter content in their applications or on their websites, and make use of our trademarks in connection with their services. If the licensees of our trademarks are not using our trademarks properly, it may limit our ability to protect our trademarks and could ultimately result in our

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trademarks being declared invalid or unenforceable. We have a policy designed to assist third parties in the proper use of our brand, trademarks and other assets, and we have an internal team dedicated to enforcing our policy and protecting our brand. Our brand protection team routinely receives and reviews reports of improper and unauthorized use of the Twitter brand, trademarks or assets and issues takedown notices or initiates discussions with the third parties to correct the issues. However, there can be no assurance that we will be able to protect against the unauthorized use of our brand, trademarks or other assets. If we fail to maintain and enforce our trademark rights, the value of our brand could be diminished. There is also a risk that one or more of our trademarks could become generic, which could result in them being declared invalid or unenforceable. For example, there is a risk that the word “Tweet” could become so commonly used that it becomes synonymous with any short comment posted publicly on the Internet, and if this happens, we could lose protection of this trademark.

We also seek to obtain patent protection for some of our technology and as of September 30, 2013, we had 9 issued U.S. patents and approximately 95 patent applications on file in the United States and abroad, although there can be no assurance that these applications will be ultimately issued as patents. We may be unable to obtain patent or trademark protection for our technologies and brands, and our existing patents and trademarks, and any patents or trademarks that may be issued in the future, may not provide us with competitive advantages or distinguish our products and services from those of our competitors. In addition, any patents and trademarks may be contested, circumvented, or found unenforceable or invalid, and we may not be able to prevent third parties from infringing, diluting or otherwise violating them. Effective protection of patent rights is expensive and difficult to maintain, both in terms of application and maintenance costs, as well as the costs of defending and enforcing those rights.

Our Innovator’s Patent Agreement, or IPA, also limits our ability to prevent infringement of our patents. In May 2013, we implemented the IPA, which we enter into with our employees and consultants, including our founders. The IPA, which applies to our current and future patents, allows us to assert our patents defensively. The IPA also allows us to assert our patents offensively with the permission of the inventors of the applicable patent. Under the IPA, an assertion of claims is considered for a defensive purpose if the claims are asserted: (i) against an entity that has filed, maintained, threatened or voluntarily participated in a patent infringement lawsuit against us or any of our users, affiliates, customers, suppliers or distributors; (ii) against an entity that has used its patents offensively against any other party in the past ten years, so long as the entity has not instituted the patent infringement lawsuit defensively in response to a patent litigation threat against the entity; or (iii) otherwise to deter a patent litigation threat against us or our users, affiliates, customers, suppliers or distributors. In addition, the IPA provides that the above limitations apply to any future owner or exclusive licensee of any of our patents, which could limit our ability to sell or license our patents to third parties. While we may be able to claim protection of our intellectual property under other rights, such as trade secrets or contractual obligations with our employees not to disclose or use confidential information, we may be unable to assert our patent rights against third parties that we believe are infringing our patents, even if such third parties are developing products and services that compete with our products and services. For example, in the event that an inventor of one of our patents leaves us for another company and uses our patented technology to compete with us, we would not be able to assert that patent against such other company unless the assertion of the patent right is for a defensive purpose. In such event, we may be limited in our ability to assert a patent right against another company, and instead would need to rely on trade secret protection or the contractual obligation of the inventor to us not to disclose or use our confidential information. In addition, the terms of the IPA could affect our ability to monetize our intellectual property portfolio.

Significant impairments of our intellectual property rights, and limitations on our ability to assert our intellectual property rights against others, could harm our business and our ability to compete. Also, obtaining, maintaining and enforcing our intellectual property rights is costly and time consuming. Any increase in the unauthorized use of our intellectual property could make it more expensive to do business and harm our operating results.

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We are currently, and expect to be in the future, party to intellectual property rights claims that are expensive and time consuming to defend, and, if resolved adversely, could have a significant impact on our business, financial condition or operating results.

Companies in the Internet, technology and media industries own large numbers of patents, copyrights, trademarks and trade secrets, and frequently enter into litigation based on allegations of infringement, misappropriation or other violations of intellectual property or other rights. Many companies in these industries, including many of our competitors, have substantially larger patent and intellectual property portfolios than we do, which could make us a target for litigation as we may not be able to assert counterclaims against parties that sue us for patent, or other intellectual property infringement. In addition, various “non-practicing entities” that own patents and other intellectual property rights often attempt to aggressively assert claims in order to extract value from technology companies. Further, from time to time we may introduce new products and services, including in areas where we currently do not have an offering, which could increase our exposure to patent and other intellectual property claims from competitors and non-practicing entities. In addition, although our standard terms and conditions for our Promoted Products and public APIs do not provide advertisers and platform partners with indemnification for intellectual property claims against them, some of our agreements with advertisers, platform partners and data partners require us to indemnify them for certain intellectual property claims against them, which could require us to incur considerable costs in defending such claims, and may require us to pay significant damages in the event of an adverse ruling. Such advertisers, platform partners and data partners may also discontinue use of our products, services and technologies as a result of injunctions or otherwise, which could result in loss of revenue and adversely impact our business.

We presently are involved in a number of intellectual property lawsuits, and as we face increasing competition and gain an increasingly high profile, we expect the number of patent and other intellectual property claims against us to grow. There may be intellectual property or other rights held by others, including issued or pending patents, that cover significant aspects of our products and services, and we cannot be sure that we are not infringing or violating, and have not infringed or violated, any third-party intellectual property rights or that we will not be held to have done so or be accused of doing so in the future. Any claim or litigation alleging that we have infringed or otherwise violated intellectual property or other rights of third parties, with or without merit, and whether or not settled out of court or determined in our favor, could be time-consuming and costly to address and resolve, and could divert the time and attention of our management and technical personnel. Some of our competitors have substantially greater resources than we do and are able to sustain the costs of complex intellectual property litigation to a greater degree and for longer periods of time than we could. The outcome of any litigation is inherently uncertain, and there can be no assurances that favorable final outcomes will be obtained in all cases. In addition, plaintiffs may seek, and we may become subject to, preliminary or provisional rulings in the course of any such litigation, including potential preliminary injunctions requiring us to cease some or all of our operations. We may decide to settle such lawsuits and disputes on terms that are unfavorable to us. Similarly, if any litigation to which we are a party is resolved adversely, we may be subject to an unfavorable judgment that may not be reversed upon appeal. The terms of such a settlement or judgment may require us to cease some or all of our operations or pay substantial amounts to the other party. In addition, we may have to seek a license to continue practices found to be in violation of a third party’s rights. If we are required, or choose to enter into royalty or licensing arrangements, such arrangements may not be available on reasonable terms, or at all, and may significantly increase our operating costs and expenses. As a result, we may also be required to develop or procure alternative non-infringing technology or discontinue use of the technology. The development or procurement of alternative non-infringing technology could require significant effort and expense or may not be feasible. An unfavorable resolution of the disputes and litigation referred to above could adversely affect our business, financial condition and operating results.

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Many of our products and services contain open source software, and we license some of our software through open source projects, which may pose particular risks to our proprietary software, products, and services in a manner that could have a negative effect on our business.

We use open source software in our products and services and will use open source software in the future. In addition, we regularly contribute software source code to open source projects under open source licenses or release internal software projects under open source licenses, and anticipate doing so in the future. The terms of many open source licenses to which we are subject have not been interpreted by U.S. or foreign courts, and there is a risk that open source software licenses could be construed in a manner that imposes unanticipated conditions or restrictions on our ability to provide or distribute our products or services. Additionally, we may from time to time face claims from third parties claiming ownership of, or demanding release of, the open source software or derivative works that we developed using such software, which could include our proprietary source code, or otherwise seeking to enforce the terms of the applicable open source license. These claims could result in litigation and could require us to make our software source code freely available, purchase a costly license or cease offering the implicated products or services unless and until we can re-engineer them to avoid infringement. This re-engineering process could require significant additional research and development resources, and we may not be able to complete it successfully. In addition to risks related to license requirements, use of certain open source software can lead to greater risks than use of third-party commercial software, as open source licensors generally do not provide warranties or controls on the origin of software. Additionally, because any software source code we contribute to open source projects is publicly available, our ability to protect our intellectual property rights with respect to such software source code may be limited or lost entirely, and we are unable to prevent our competitors or others from using such contributed software source code. Any of these risks could be difficult to eliminate or manage, and, if not addressed, could have a negative effect on our business, financial condition and operating results.

We may expend substantial funds in connection with the tax liabilities that arise upon the initial settlement of RSUs in connection with this offering, and the manner in which we fund that expenditure may have an adverse effect on our financial condition.

We may expend substantial funds to satisfy tax withholding and remittance obligations when we settle a portion of our RSUs granted prior to the date of this prospectus. Pre-2013 RSUs vest upon the satisfaction of both a service condition and a performance condition. The service condition for the majority of the Pre-2013 RSUs is satisfied over a period of four years. The performance condition in connection with our Pre-2013 RSUs will be satisfied on the earlier of (i) the date that is the earlier of (x) six months after the effective date of this offering or (y) March 8th of the calendar year following the effective date of this offering (which we may elect to accelerate to February 15th); and (ii) the date of a change in control. On the settlement dates for the Pre-2013 RSUs, we may choose to allow our employees who are not executive officers to sell shares of our common stock received upon the vesting and settlement of Pre-2013 RSUs in the public market to satisfy their income tax obligations related to the vesting and settlement of such awards, or we may withhold shares and remit income taxes on behalf of the holders of the Pre-2013 RSUs at the applicable minimum statutory rates, which we refer to as a net settlement. We expect the applicable minimum statutory rates to be approximately 40% on average, and the income taxes due would be based on the then-current value of the underlying shares of our common stock. Based on the number of Pre-2013 RSUs outstanding as of September 30, 2013 for which the service condition had been satisfied on that date, and assuming (i) the performance condition had been satisfied on that date and (ii) that the price of our common stock at the time of settlement was equal to $        , which is the midpoint of the estimated offering price range set forth on the cover page of this prospectus, we estimate that this tax obligation on the initial settlement date would be approximately $         in the aggregate. The amount of this obligation could be higher or lower, depending on the price of shares of our common stock on the initial settlement date for

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the Pre-2013 RSUs. To settle these Pre-2013 RSUs on the initial settlement date, assuming a 40% tax withholding rate, if we choose to undertake a net settlement of all of these awards rather than allowing our employees who are not executive officers to sell shares of our common stock in the public market to satisfy their income tax obligations related to the vesting and settlement of Pre-2013 RSUs, we would expect to deliver an aggregate of approximately              shares of our common stock to Pre-2013 RSU holders and withhold an aggregate of approximately              shares of our common stock. In connection with these net settlements, we would withhold and remit the tax liabilities on behalf of the Pre-2013 RSU holders to the relevant tax authorities in cash.

If we choose to undertake a net settlement of our Pre-2013 RSUs, then in order to fund the tax withholding and remittance obligations on behalf of our Pre-2013 RSU holders, we would expect to use a substantial portion of our cash and cash equivalent balances, or, alternatively, we may choose to borrow funds or a combination of cash and borrowed funds to satisfy these obligations.

We may require additional capital to support our operations or the growth of our business, and we cannot be certain that this capital will be available on reasonable terms when required, or at all.

From time to time, we may need additional financing to operate or grow our business. Our ability to obtain additional financing, if and when required, will depend on investor and lender demand, our operating performance, the condition of the capital markets and other factors, and we cannot assure you that additional financing will be available to us on favorable terms when required, or at all. If we raise additional funds through the issuance of equity, equity-linked or debt securities, those securities may have rights, preferences or privileges senior to the rights of our common stock, and our existing stockholders may experience dilution. If we are unable to obtain adequate financing or financing on terms satisfactory to us when we require it, our ability to continue to support the operation or growth of our business could be significantly impaired and our operating results may be harmed.

We rely on assumptions and estimates to calculate certain of our key metrics, and real or perceived inaccuracies in such metrics may harm our reputation and negatively affect our business.

The numbers of our active users and timeline views are calculated using internal company data that has not been independently verified. While these numbers are based on what we believe to be reasonable calculations for the applicable period of measurement, there are inherent challenges in measuring usage and user engagement across our large user base around the world. For example, there are a number of false or spam accounts in existence on our platform. We currently estimate that false or spam accounts represent less than 5% of our MAUs. However, this estimate is based on an internal review of a sample of accounts and we apply significant judgment in making this determination. As such, our estimation of false or spam accounts may not accurately represent the actual number of such accounts, and the actual number of false or spam accounts could be higher than we have currently estimated. We are continually seeking to improve our ability to estimate the total number of spam accounts and eliminate them from the calculation of our active users, but we otherwise treat multiple accounts held by a single person or organization as multiple users for purposes of calculating our active users because we permit people and organizations to have more than one account. Additionally, some accounts used by organizations are used by many people within the organization. As such, the calculations of our active users may not accurately reflect the actual number of people or organizations using our platform.

Our metrics are also affected by mobile applications that automatically contact our servers for regular updates with no user action involved, and this activity can cause our system to count the user associated with such a device as an active user on the day such contact occurs. The calculations of MAUs presented in this prospectus may be affected by this activity. The impact of this automatic activity

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on our metrics varies by geography because mobile application usage varies in different regions of the world. In addition, our data regarding user geographic location is based on the IP address associated with the account when a user initially registered the account on Twitter. The IP address may not always accurately reflect a user’s actual location at the time of user engagement on our platform.

We present and discuss timeline views in the nine months ended September 30, 2012, but we did not track all of the timeline views on our mobile applications during the three months ended March 31, 2012. We have included in this prospectus estimates for actual timeline views in the three months ended March 31, 2012 for the mobile applications we did not track. In addition, we have estimated a small percentage of the timeline views in the three months ended September 30, 2013 to account for certain timeline views that were logged incorrectly during the quarter as a result of a product update. We believe these estimates to be reasonable, but actual numbers could differ from our estimates. Further, timeline views in the nine months ended September 30, 2012 exclude an immaterial number of timeline views in our mobile applications, certain of which were not fully tracked until June 2012.

We regularly review and may adjust our processes for calculating our internal metrics to improve their accuracy. Our measures of user growth and user engagement may differ from estimates published by third parties or from similarly-titled metrics of our competitors due to differences in methodology. If advertisers, platform partners or investors do not perceive our user metrics to be accurate representations of our user base or user engagement, or if we discover material inaccuracies in our user metrics, our reputation may be harmed and advertisers and platform partners may be less willing to allocate their budgets or resources to our products and services, which could negatively affect our business and operating results.

We depend on highly skilled personnel to grow and operate our business, and if we are unable to hire, retain and motivate our personnel, we may not be able to grow effectively.

Our future success will depend upon our continued ability to identify, hire, develop, motivate and retain highly skilled personnel, including senior management, engineers, designers and product managers. Our ability to execute efficiently is dependent upon contributions from our employees, in particular our senior management team. We do not have employment agreements other than offer letters with any member of our senior management or other key employee, and we do not maintain key person life insurance for any employee. In addition, from time to time, there may be changes in our senior management team that may be disruptive to our business. If our senior management team, including any new hires that we may make, fails to work together effectively and to execute our plans and strategies on a timely basis, our business could be harmed.

Our growth strategy also depends on our ability to expand and retain our organization with highly skilled personnel. Identifying, recruiting, training and integrating qualified individuals will require significant time, expense and attention. In addition to hiring new employees, we must continue to focus on retaining our best employees. Many of our employees may be able to receive significant proceeds from sales of our equity in the public markets after this offering, which may reduce their motivation to continue to work for us. Competition for highly skilled personnel is intense, particularly in the San Francisco Bay Area, where our headquarters is located. We may need to invest significant amounts of cash and equity to attract and retain new employees and we may never realize returns on these investments. If we are not able to effectively add and retain employees, our ability to achieve our strategic objectives will be adversely impacted, and our business will be harmed.

Our corporate culture has contributed to our success, and if we cannot maintain this culture as we grow, we could lose the innovation, creativity and teamwork fostered by our culture, and our business may be harmed.

We believe that our culture has been and will continue to be a key contributor to our success. From January 1, 2010 to September 30, 2013, we increased the size of our workforce by more than

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2,200 employees, and we expect to continue to hire aggressively as we expand. If we do not continue to develop our corporate culture or maintain our core values as we grow and evolve, we may be unable to foster the innovation, creativity and teamwork we believe we need to support our growth. Moreover, liquidity available to our employee securityholders following this offering could lead to disparities of wealth among our employees, which could adversely impact relations among employees and our culture in general. Our transition from a private company to a public company may result in a change to our corporate culture, which could harm our business.

We rely in part on application marketplaces and Internet search engines to drive traffic to our products and services, and if we fail to appear high up in the search results or rankings, traffic to our platform could decline and our business and operating results could be adversely affected.

We rely on application marketplaces, such as Apple’s App Store and Google’s Play, to drive downloads of our mobile applications. In the future, Apple, Google or other operators of application marketplaces may make changes to their marketplaces which make access to our products and services more difficult. We also depend in part on Internet search engines, such as Google, Bing and Yahoo!, to drive traffic to our website. For example, when a user types an inquiry into a search engine, we rely on a high organic search result ranking of our webpages in these search results to refer the user to our website. However, our ability to maintain high organic search result rankings is not within our control. Our competitors’ search engine optimization, or SEO, efforts may result in their websites receiving a higher search result page ranking than ours, or Internet search engines could revise their methodologies in a way that would adversely affect our search result rankings. For example, Google has integrated its social networking offerings, including Google+, with certain of its products, including search, which has negatively impacted the organic search ranking of our webpages. If Internet search engines modify their search algorithms in ways that are detrimental to us, or if our competitors’ SEO efforts are more successful than ours, the growth in our user base could slow. Our website has experienced fluctuations in search result rankings in the past, and we anticipate similar fluctuations in the future. Any reduction in the number of users directed to our mobile applications or website through application marketplaces and search engines could harm our business and operating results.

More people are using devices other than personal computers to access the Internet and new platforms to produce and consume content, and we need to continue to promote the adoption of our mobile applications, and our business and operating results may be harmed if we are unable to do so.

The number of people who access the Internet through devices other than personal computers, including mobile phones, smartphones, handheld computers such as net books and tablets, video game consoles and television set-top devices, has increased dramatically in the past few years. In the three months ended September 30, 2013, over 70% of our advertising revenue was generated from mobile devices. Since we generate a majority of our advertising revenue through users on mobile devices, we must continue to drive adoption of our mobile applications. In addition, mobile users frequently change or upgrade their mobile devices. Our business and operating results may be harmed if our users do not install our mobile application when they change or upgrade their mobile device. Although we generate the majority of our advertising revenue from ad engagements on mobile devices, certain of our products and services, including Promoted Trends and Promoted Accounts, receive less prominence on our mobile applications than they do on our desktop applications. This has in the past reduced, and may in the future continue to reduce, the amount of revenue we are able to generate from these products and services as users increasingly access our products and services through mobile and alternative devices. In addition, as new devices and platforms are continually being released, users may consume content in a manner that is more difficult to monetize. It is difficult to predict the problems we may encounter in adapting our products and services and developing competitive new products and services that are compatible with new devices or platforms. If we are

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unable to develop products and services that are compatible with new devices and platforms, or if we are unable to drive continued adoption of our mobile applications, our business and operating results may be harmed.

Future acquisitions and investments could disrupt our business and harm our financial condition and operating results.

Our success will depend, in part, on our ability to expand our products and services, and grow our business in response to changing technologies, user and advertiser demands, and competitive pressures. In some circumstances, we may determine to do so through the acquisition of complementary businesses and technologies rather than through internal development, including, for example, our recent acquisitions of Vine Labs, Inc., a mobile application that enables users to create and distribute videos that are up to six seconds in length, and Bluefin Labs, Inc., a social television analytics company that provides data products to brand advertisers, agencies and television networks. We also recently entered into a definitive agreement to acquire MoPub, a mobile-focused advertising exchange, which we currently expect to close in November 2013. The identification of suitable acquisition candidates can be difficult, time-consuming and costly, and we may not be able to successfully complete identified acquisitions. The risks we face in connection with acquisitions include:

Our failure to address these risks or other problems encountered in connection with our past or future acquisitions and investments could cause us to fail to realize the anticipated benefits of these acquisitions or investments, cause us to incur unanticipated liabilities, and harm our business generally. Future acquisitions could also result in dilutive issuances of our equity securities, the incurrence of debt, contingent liabilities, amortization expenses, incremental operating expenses or the write-off of goodwill, any of which could harm our financial condition or operating results.

If we fail to maintain an effective system of disclosure controls and internal control over financial reporting, our ability to produce timely and accurate financial statements or comply with applicable regulations could be impaired.

As a public company, we will be subject to the reporting requirements of the Securities Exchange Act of 1934, as amended, or the Exchange Act, the Sarbanes-Oxley Act of 2002, or the Sarbanes-

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Oxley Act, and the listing standards of the New York Stock Exchange. We expect that the requirements of these rules and regulations will continue to increase our legal, accounting and financial compliance costs, make some activities more difficult, time consuming and costly, and place significant strain on our personnel, systems and resources.

The Sarbanes-Oxley Act requires, among other things, that we maintain effective disclosure controls and procedures and internal control over financial reporting. We are continuing to develop and refine our disclosure controls and other procedures that are designed to ensure that information required to be disclosed by us in the reports that we will file with the SEC is recorded, processed, summarized and reported within the time periods specified in SEC rules and forms, and that information required to be disclosed in reports under the Exchange Act is accumulated and communicated to our principal executive and financial officers. We are also continuing to improve our internal control over financial reporting. In order to maintain and improve the effectiveness of our disclosure controls and procedures and internal control over financial reporting, we have expended, and anticipate that we will continue to expend, significant resources, including accounting-related costs and significant management oversight.

Our current controls and any new controls that we develop may become inadequate because of changes in conditions in our business. Further, weaknesses in our disclosure controls or our internal control over financial reporting may be discovered in the future. Any failure to develop or maintain effective controls, or any difficulties encountered in their implementation or improvement, could harm our operating results or cause us to fail to meet our reporting obligations and may result in a restatement of our financial statements for prior periods. Any failure to implement and maintain effective internal control over financial reporting also could adversely affect the results of management evaluations and independent registered public accounting firm audits of our internal control over financial reporting that we will eventually be required to include in our periodic reports that will be filed with the SEC. Ineffective disclosure controls and procedures and internal control over financial reporting could also cause investors to lose confidence in our reported financial and other information, which would likely have a negative effect on the trading price of our common stock. In addition, if we are unable to continue to meet these requirements, we may not be able to remain listed on the New York Stock Exchange.

We are not currently required to comply with the SEC rules that implement Section 404 of the Sarbanes-Oxley Act, and are therefore not required to make a formal assessment of the effectiveness of our internal control over financial reporting for that purpose. Upon becoming a public company, we will be required to provide an annual management report on the effectiveness of our internal control over financial reporting commencing with our second annual report on Form 10-K. Our independent registered public accounting firm is not required to audit the effectiveness of our internal control over financial reporting until after we are no longer an “emerging growth company,” as defined in the Jumpstart Our Business Startups Act of 2012, or the JOBS Act. At such time, our independent registered public accounting firm may issue a report that is adverse in the event it is not satisfied with the level at which our internal control over financial reporting is documented, designed or operating.

Any failure to maintain effective disclosure controls and internal control over financial reporting could have a material and adverse effect on our business and operating results, and cause a decline in the price of our common stock.

We are an emerging growth company, and any decision on our part to comply only with certain reduced reporting and disclosure requirements applicable to emerging growth companies could make our common stock less attractive to investors.

We are an emerging growth company, and, for as long as we continue to be an emerging growth company, we may choose to take advantage of exemptions from various reporting requirements

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applicable to other public companies but not to “emerging growth companies,” including, but not limited to, not being required to have our independent registered public accounting firm audit our internal control over financial reporting under Section 404 of the Sarbanes-Oxley Act, reduced disclosure obligations regarding executive compensation in our periodic reports and proxy statements, and exemptions from the requirements of holding a nonbinding advisory vote on executive compensation and stockholder approval of any golden parachute payments not previously approved. We could be an emerging growth company for up to five years following the completion of this offering. We will cease to be an emerging growth company upon the earliest of: (i) the end of the fiscal year following the fifth anniversary of this offering, (ii) the first fiscal year after our annual gross revenue are $1.0 billion or more, (iii) the date on which we have, during the previous three-year period, issued more than $1.0 billion in non-convertible debt securities or (iv) the end of any fiscal year in which the market value of our common stock held by non-affiliates exceeded $700 million as of the end of the second quarter of that fiscal year. We cannot predict if investors will find our common stock less attractive if we choose to rely on these exemptions. If some investors find our common stock less attractive as a result of any choices to reduce future disclosure, there may be a less active trading market for our common stock and the price of our common stock may be more volatile.

Under the JOBS Act, emerging growth companies can also delay adopting new or revised accounting standards until such time as those standards apply to private companies. We have irrevocably elected not to avail ourselves of this accommodation allowing for delayed adoption of new or revised accounting standards, and, therefore, we will be subject to the same new or revised accounting standards as other public companies that are not emerging growth companies.

If currency exchange rates fluctuate substantially in the future, our operating results, which are reported in U.S. dollars, could be adversely affected.

As we continue to expand our international operations, we will become more exposed to the effects of fluctuations in currency exchange rates. We incur expenses for employee compensation and other operating expenses at our international locations in the local currency, and accept payment from advertisers or data partners in currencies other than the U.S. dollar. Since we conduct business in currencies other than U.S. dollars but report our operating results in U.S. dollars, we face exposure to fluctuations in currency exchange rates. Consequently, exchange rate fluctuations between the U.S. dollar and other currencies could have a material impact on our operating results.

Our business is subject to the risks of earthquakes, fire, power outages, floods and other catastrophic events, and to interruption by man-made problems such as terrorism.

A significant natural disaster, such as an earthquake, fire, flood or significant power outage could have a material adverse impact on our business, operating results, and financial condition. Our headquarters and certain of our co-located data center facilities are located in the San Francisco Bay Area, a region known for seismic activity. Despite any precautions we may take, the occurrence of a natural disaster or other unanticipated problems at our data centers could result in lengthy interruptions in our services. In addition, acts of terrorism and other geo-political unrest could cause disruptions in our business. All of the aforementioned risks may be further increased if our disaster recovery plans prove to be inadequate. We have implemented a disaster recovery program, which allows us to move production to a back-up data center in the event of a catastrophe. Although this program is functional, we do not currently serve network traffic equally from each data center, so if our primary data center shuts down, there will be a period of time that our products or services, or certain of our products or services, will remain inaccessible to our users or our users may experience severe issues accessing our products and services.

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We do not carry business interruption insurance sufficient to compensate us for the potentially significant losses, including the potential harm to our business that may result from interruptions in our ability to provide our products and services.

We may have exposure to greater than anticipated tax liabilities, which could adversely impact our operating results.

Our income tax obligations are based in part on our corporate operating structure, including the manner in which we develop, value and use our intellectual property and the scope of our international operations. The tax laws applicable to our international business activities, including the laws of the United States and other jurisdictions, are subject to interpretation. The taxing authorities of the jurisdictions in which we operate may challenge our methodologies for valuing developed technology (or other intangible assets) or intercompany arrangements, which could increase our worldwide effective tax rate and harm our financial condition and operating results. We are subject to review and audit by U.S. federal and state and foreign tax authorities. Tax authorities may disagree with certain positions we have taken and any adverse outcome of such a review or audit could have a negative effect on our financial position and operating results. In addition, our future income taxes could be adversely affected by earnings being lower than anticipated in jurisdictions that have lower statutory tax rates and higher than anticipated in jurisdictions that have higher statutory tax rates, by changes in the valuation of our deferred tax assets and liabilities, or by changes in tax laws, regulations or accounting principles. Tax expenses, or disputes with tax authorities, could adversely impact our operating results.

If our goodwill or intangible assets become impaired, we may be required to record a significant charge to earnings.

Under GAAP, we review our intangible assets for impairment when events or changes in circumstances indicate the carrying value may not be recoverable. Goodwill is required to be tested for impairment at least annually. As of September 30, 2013, we had recorded a total of $183.1 million of goodwill and intangible assets related to our acquisitions. An adverse change in market conditions, particularly if such change has the effect of changing one of our critical assumptions or estimates, could result in a change to the estimation of fair value that could result in an impairment charge to our goodwill or intangible assets. Any such material charges may have a material negative impact on our operating results.

Our ability to use our net operating loss carryforwards and certain other tax attributes may be limited.

As of December 31, 2012, we had U.S. federal net operating loss carryforwards of approximately $298.8 million and state net operating loss carryforwards of approximately $216.7 million. Under Sections 382 and 383 of Internal Revenue Code of 1986, as amended, or the Code, if a corporation undergoes an “ownership change,” the corporation’s ability to use its pre-change net operating loss carryforwards and other pre-change tax attributes, such as research tax credits, to offset its post-change income and taxes may be limited. In general, an “ownership change” occurs if there is a cumulative change in our ownership by “5% shareholders” that exceeds 50 percentage points over a rolling three-year period. Similar rules may apply under state tax laws. In the event that it is determined that we have in the past experienced an ownership change, or if we experience one or more ownership changes as a result of this offering or future transactions in our stock, then we may be limited in our ability to use our net operating loss carryforwards and other tax assets to reduce taxes owed on the net taxable income that we earn. Any such limitations on the ability to use our net operating loss carryforwards and other tax assets could adversely impact our business, financial condition and operating results.

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Risks Related to Ownership of Our Common Stock and this Offering

Existing executive officers, directors and holders of 5% or more of our common stock will collectively beneficially own    % of our common stock and continue to have substantial control over us after this offering, which will limit your ability to influence the outcome of important transactions, including a change in control.

Our directors, executive officers and each of our stockholders who own 5% or more of our outstanding common stock and their affiliates, in the aggregate, will beneficially own approximately    % of the outstanding shares of our common stock after this offering, based on the number of shares outstanding as of September 30, 2013. As a result, these stockholders, if acting together, will be able to influence or control matters requiring approval by our stockholders, including the election of directors and the approval of mergers, acquisitions or other extraordinary transactions. They may also have interests that differ from yours and may vote in a way with which you disagree and which may be adverse to your interests. This concentration of ownership may have the effect of delaying, preventing or deterring a change in control of our company, could deprive our stockholders of an opportunity to receive a premium for their common stock as part of a sale of our company and might ultimately affect the market price of our common stock.