The Massive Open Online Courses (MOOC) have experienced rapid development. However, high dropout rate has become a salient issue. Many studies have attempted to understand this phenomenon; other have explored mechanisms for enhancing retention. For instance, social media has been used to improve student engagement and retention. However there is a lack of (1) empirical studies of social media use and engagement compared to embedded MOOC forums; and (2) rationales for social media use from both instructors' and students' perspectives. We addressed these open issues through the collection and analysis of real usage data from three MOOC forums and their associated social media (i.e., Facebook) groups as well as conducting interviews of instructors and students. We found that students show higher engagement and retention in social media than in MOOC forums, and identified both instructors' and students' perspectives that lead to the results. We discuss design implications for future MOOC platforms.