The use of social media is rapidly expanding and offers a new way for public health information to be communicated to the broader population. Based on a systematic literature review, this briefing note summarizes the evidence for public health groups considering the use of social media. The paper begins by defining social media, followed by a discussion of how social media can be used in public health, how it has been effectively applied, and its major contributions to public health. It then addresses the key questions: ‘What is the effectiveness of social media?' and ‘What is the impact on health inequalities?'