Background: Social networking technologies are an emerging tool for HIV prevention. Objective: To determine whether social networking communities can increase HIV testing among African American and Latino men who have sex with men (MSM). Design: Randomized, controlled trial with concealed allocation. (ClinicalTrials.gov: NCT01701206) Setting: Online. Patients: 112 MSM based in Los Angeles, more than 85% of whom were African American or Latino. Intervention: Sixteen peer leaders were randomly assigned to deliver information about HIV or general health to participants via Facebook groups over 12 weeks. After participants accepted a request to join the group, participation was voluntary. Group participation and engagement were monitored. Participants could request a free, home-based HIV testing kit and completed questionnaires at baseline and 12-week follow-up. Measurements: Participant acceptance of and engagement in the intervention and social network participation, rates of home-based HIV testing, and sexual risk behaviors. Results: Almost 95% of intervention participants and 73% of control participants voluntarily communicated using the social platform. Twenty-five of 57 intervention participants (44%) requested home-based HIV testing kits compared with 11 of 55 control participants (20%) (difference, 24 percentage points [95% CI, 8 to 41 percentage points]). Nine of the 25 intervention participants (36%) who requested the test took it and mailed it back compared with 2 of the 11 control participants (18%) who requested the test. Retention at study follow-up was more than 93%. Limitation: Only 2 Facebook communities were included for each group. Conclusion: Social networking communities are acceptable and effective tools to increase home-based HIV testing among at-risk populations. Primary Funding Source: National Institute of Mental Health. © 2013 American College of Physicians.