Use of online social networking technologies has been growing rapidly, especially among groups at risk for sexually transmitted infections (STIs), such as men who have sex with men (MSM), homeless youth, and African American and Latino populations. International studies are beginning to suggest a number of important relationships between social networking technologies, at-risk populations, and risk for sexually transmitted infections: (1) At-risk populations are using social networking technologies to meet sexual partners, (2) Although at-risk populations who use social networking technologies are engaging in high rates of sexual intercourse, they are engaging in prevention behaviours that might mitigate their risk, and (3) Social networking technologies can be used as platforms for scaling and diffusing STI prevention and testing interventions. Online social networks are tools that can be used to rapidly spread information and social norms. These same technologies that can be used to potentially spread STI transmission must also be implemented as tools for preventing transmission. Recent research is discussed.