This thesis looks at how social media in China are involved in online activism under different social, technological and political conditions. This line of enquiry is driven by the historical shift observed in China over the past few years, with contentious episodes of online mass incidents frequently taking place from 2010 to 2012, but declining significantly in the years since then. Studying this phenomenon is not only crucial to understanding contemporary China, but also interesting from a more general and theoretical perspective as the local occurrences are reflective of a global phenomenon: the nexus between social media and politics. Despite increasing scholarly interest in this topic, most studies have considered social media as a neutral technological tool, either for Internet users to disseminate information and create networks, or for the Chinese state to censor and monitor online activities. I argue that the dynamics of social media activism in China cannot be understood without a complex engagement with the technological affordances and commercial logic of social media platforms, the extensive but constantly evolving forms of online censorship, the patterns of collective action, and the Chinese society. Adopting this interdisciplinary perspective, the thesis analyses online mass incidents in China from 2010 to 2016, focusing on a number of incidents on Weibo and WeChat, the two most widely used Chinese social media platforms. I consider how these platforms are involved in forming online mass incidents and how their roles are shaped by the convergence of social, technological, political and commercial factors in different historical moments. This thesis proposes approaching online mass incidents as a particular manifestation of contemporary online crowds and utilising the notion of visibility as a central concept in analysing crowd formation. I look at how different actors, not only social media users, but also commercial online platforms and the state, are producing, competing and negotiating for what should be and how they can become visible in and through online contention. Empirically, I draw from 33 interviews with activists, platform company staff and online opinion leaders, as well as observing activists' WeChat groups and undertaking content analysis of relevant social media texts. This thesis demonstrates that online mass incidents are far from being completely spontaneous; rather, they are highly complex episodes of online contention that emerge when a number of grievances meet with temporary openings in the online public sphere and find actors willing to recognise and amplify their visibility. The findings of this research contribute to our understanding of how Chinese social media activism is articulated through the interaction between activists, platforms and the state, and they shed important light on challenges and prospects for future activism in China.