Social media are potentially valuable tools for disseminating cancer education messages, but the differential effects of various sites on persuasive outcomes are unknown. In an effort to inform future health promotion, this research tested the effects of Facebook, YouTube, Twitter, and blogs for delivering a cancer risk reduction message. Using an experimental design, participants were randomly placed in several conditions that delivered the same message but with different forms of social media. Effects on comprehension and attitudes were examined, as they are important variables in the behavior change process. YouTube led to higher comprehension and stronger attitudes toward cancer risk reduction than Twitter, but there were no differences between other sites. Additionally, YouTube led to stronger attitudes toward cancer risk reduction as compared to Facebook, but not any other sites. These results demonstrate that, even if the message is kept constant, the form of social media used to deliver content can have an effect on persuasive outcomes. More research is needed to determine the mechanisms behind the differences found, however. Altogether, this line of research is valuable for any individuals seeking to use social media for health promotion purposes and could have direct implications for the development of cancer risk reduction campaigns.