Little is known about how social media are used in cancer care. We conducted a systematic review of the use and taxonomy of social media in cancer-related studies, in PubMed, Web of Knowledge, CINAHL, and Google Scholar. We located 1350 articles published through October 2013; 69 met study inclusion criteria. Early research (1996-2007) was predominantly descriptive studies of online forums. Later, researchers began analyzing blogs, videos shared on YouTube, and social networking sites. Most studies (n = 62) were descriptive, and only 7 reported intervention studies published since 2010. Future research should include more intervention studies to determine how social media can influence behavior, and more empirical research is needed on how social media may be used to reduce health disparities.