Self-presentation, the process by which people disclose information about themselves to others, is fundamental to online interaction and research on communication technology. Technology often mediates the self-presentation process by obscuring who is in the audience via constrained cues and opaque feed algorithms that govern the visibility of social media content. This can make it risky to disclose sensitive or potentially stigmatizing information about oneself, because it could fall into the wrong hands or be seen by an unsupportive audience. Still, there are times when it is socially beneficial to disclose sensitive information, such as LGBTQ+ (lesbian, gay, bisexual, transgender, queer, and others) people expressing their identities or disclosing HIV status. Decisions about sensitive disclosure, moreover, can be even more complicated in today’s social media landscape with many platforms and audiences in play, particularly for younger users who often use many platforms. We lack a good understanding, however, of how people make these decisions. This article addresses questions about sensitive disclosure on social media through a survey study of adolescent men who have sex with men and their willingness to disclose on social media the use of pre-exposure prophylaxis (PrEP), an HIV prevention medication. Results suggest that perceived platform audience composition and platform features such as ephemerality play into disclosure decisions, as well as the perceived normativity of PrEP use among peers.