PURPOSE: This article reports findings from an evaluation of reach and engagement of The FaceSpace Project, a novel sexual health promotion project delivered through social networking sites that targeted young people aged 16-29 years. METHODS: Multiple methods were used to evaluate project reach and engagement. The evaluation focussed on quantitative data (online usage statistics, online surveys), complemented by available qualitative data (project team meeting notes). RESULTS: The project reached 900 fans who were mostly between 18 and 34 years of age. The most successful ways of increasing audience reach were via Facebook advertisements and tagging photos of young people attending a music festival on the project Facebook page. Peaks in Facebook page interactions (comments and “likes”) coincided with recruitment peaks and when videos were posted. However, video views varied greatly between postings. Feedback from the project team for increasing engagement in future social networking site interventions included having one centralized Facebook page and using episodic videos. CONCLUSIONS: This evaluation is among the first to assess the use of social networking sites for sexual health promotion and provides information to inform the implementation and evaluation of future projects using new media. Social networking sites offer great potential to reach and engage young people for sexual health promotion. However, further work is required to improve implementation and promote audience reach and engagement as well as to determine effectiveness of social networking sites in changing knowledge, attitudes, and behaviors.