Social media such as Facebook and Twitter are among the most popular types of media platforms designed for computer-mediated social interaction. However, the research findings on their social and psychological effects have been inconsistent and controversial. Some researchers suggest social media use enhances users’ well-being while reducing ill-being (e.g., loneliness); others indicate that heavy use of social media use can exacerbate users’ existing problems or even create new ones (e.g., addictive or unregulated use). This chapter is an effort to provide a synthesized and systematic overview of this fast growing domain of research. Specifically, this chapter deals with: (1) who uses social media and why, (2) the link between social media use and psychosocial well-being, (3) enhancement versus exacerbation effects of social media on users’ existing psychosocial elements, and (4) future research suggestions for social media use and well-being.