This article starts with an overview on China's MOOC phenomenon and social media, and then reports a comparative, multiple case study on three selected MOOC communities that have emerged on social media in China. These representative MOOC communities included: (a) MOOC Academy, the largest MOOC community in China, (b) Zhejiang University of Technology MOOC Group, the first officially registered student MOOC organization in China's universities, and (c) a fully online community devoted for high school (or younger) students in MOOCs. Data were collected through a variety of Chinese social media (e.g., Sina Weibo, QQ Qun, and WeChat), social networking sites (e.g., Baidu Tieba, GuoKr, Sina blog, etc.), news press, and Web-based community portals. Text mining and content analyses were conducted to study the varied MOOC communities and how they functioned on social media to promote MOOCs as well as to support MOOC learners and the broader community. Implications for research and practice of MOOC communities are discussed.