Background As efforts continue to increase rates of HIV testing and condom use among at-risk communities in England, organisations have sought use social media for health promotion interventions. As part of a wider evaluation of It Starts With Me (ISWM), a sexual health promotion intervention in England targeting gay and bisexual men and African people through Facebook, this study sought to explore how the online environment shapes end user engagement with sexual health interventions. A primary objective was to explore how privacy concerns can act as a barrier to engagement for the audience of ISWM. Methods: A purposive sample of 40 individuals were recruited, who were targeted by the intervention for in-depth interviews. Data collection was in two phases. In the first phase, individuals were sampled based on engagement with online health interventions in general, while in the second phase, all individuals were sampled on the basis of engagement with the intervention. Results: Privacy concerns related to the ecology of social networking sites, issues with implied disclosure and discrimination, as well as uncertainty over control of data. These concerns limited the organic reach of the intervention by confining the intervention to those who already held the norms diffused through it, and by discouraging participants from sharing and commenting on content. Conclusions: Care should be taken to address concerns when designing interventions delivered through social media. Gated interventions may be more beneficial for marginalised communities, while large-scale interventions such as ISWM may provide a useful backdrop for face-to-face interventions.