© Danielle E Ramo, Johannes Thrul, Kathryn Chavez, Kevin L Delucchi, Judith J Prochaska. Background: Young adult smokers are a challenging group to engage in smoking cessation interventions. With wide reach and engagement among users, Facebook offers opportunity to engage young people in socially supportive communities for quitting smoking and sustaining abstinence. Objective: We developed and tested initial efficacy, engagement, and acceptability of the Tobacco Status Project, a smoking cessation intervention for young adults delivered within Facebook. Methods: The intervention was based on the US Public Health Service Clinical Practice Guidelines and the Transtheoretical Model and enrolled participants into study-run 3-month secret Facebook groups matched on readiness to quit smoking. Cigarette smokers (N=79) aged 18-25, who used Facebook on most days, were recruited via Facebook. All participants received the intervention and were randomized to one of three monetary incentive groups tied to engagement (commenting in groups). Assessments were completed at baseline, 3-, 6-, and 12-months follow-up. Analyses examined retention, smoking outcomes over 12 months (7-day point prevalence abstinence, .50% reduction in cigarettes smoked, quit attempts and strategies used, readiness to quit), engagement, and satisfaction with the intervention. Results: Retention was 82% (65/79) at 6 months and 72% (57/79) at 12 months. From baseline to 12-months follow-up, there was a significant increase in the proportion prepared to quit (10/79, 13%; 36/79, 46%, P<.001). Over a third (28/79, 35%) reduced their cigarette consumption by 50% or greater, and 66% (52/79) made at least one 24-hour quit attempt during the study. In an intent-to-treat analysis, 13% (10/79) self-reported 7-day abstinence (6/79, 8% verified biochemically) at 12-months follow-up. In their quit attempts, 11% (9/79) used a nicotine replacement therapy approved by the Food and Drug Administration, while 18% (14/79) used an electronic nicotine delivery system to quit (eg, electronic cigarette). A majority (48/79, 61%) commented on at least one Facebook post, with more commenting among those with biochemically verified abstinence at 3 months (P=.036) and those randomized to receive a personal monetary incentive (P=.015). Over a third of participants (28/79, 35%) reported reading most or all of the Facebook posts. Highest acceptability ratings of the intervention were for post ease (57/79, 72%) and thinking about what they read (52/79, 66%); 71% (56/79) recommended the program to others. Only 5 participants attended the optional cognitive-behavioral counseling sessions, though their attendance was high (6/7 sessions overall) and the sessions were rated as easy to understand, useful, and helpful (all 90-100% agreed). Conclusions: A Facebook quit smoking intervention is attractive and feasible to deliver, and early efficacy data are encouraging. However, the 1.5-fold greater use of electronic cigarettes over nicotine replacement products for quitting is concerning.