Abstract: This paper examines the role of Chinese social media in three areas: organizing collective action, surveillance of government officials, and propaganda. Our study is based on a data set of 13.2 billion blog posts published on Sina Weibo -- the most prominent Chinese microblogging platform -- during the 2009-2013 period. We find millions of posts discussing explicit corruption allegations and collective action events, such as, protests, strikes, and demonstrations. More intensive use of Sina Weibo is significantly associated with higher incidence of protests and large-scale conflicts. We also find that social media are effective tools for surveillance: Sina Weibo content predicts collective action events one day before their occurrence and corruption charges one year in advance. Finally, we estimate that our data contain 600,000 government-affiliated accounts which contribute 4% of all posts about political and economical issues on Sina Weibo. The share of government accounts is larger in areas with a higher level of internet censorship and where newspapers have a stronger pro-government bias. Overall, our findings suggest that the Chinese government regulates social media to balance threats to regime stability against the benefits of utilizing bottom-up information.