This paper reports on the use of social network sites (MySpace and Facebook) by homeless young people, an extraordinary user population, made so in part by its vulnerability. Twenty-three participants of diverse ethnicities, 11 women and 12 men (mean age, 21.7 years), were interviewed in same-sex discussion groups of four participants each. The interviews consisted of questions about the uses, benefits, and harms of social network sites and how people present themselves online. Qualitative analysis of the discussion group transcripts shows how young people explore their identities, cultivate and exploit social ties, experience interpersonal tensions, manage incompatible audiences, and respond to shifting affiliations and transitions. From this analysis, implications for social intervention and technical design are presented, focused on maintaining ties with pro-social family and friends and with maintaining separation between communication spheres of incompatible audiences. This work contributes to the growing literature on vital, deeply human experiences that have become associated with social network sites.