Background:
Tobacco smoking causes multiple diseases, premature mortality and is a major public health threat. In Switzerland, smoking prevalence is high (25%) but stable since 2011. Half of smokers want to quit but they underuse effective smoking cessation services. Programs using social networks are a new smoking cessation approach potentially able to reach many smokers. We designed, implemented and tested an innovative large-scale smoking cessation program using Facebook for 7 months in 2016 in 4 states of Switzerland.

Methods:
7008 smokers were enrolled within 2 months through Facebook advertising. The program created 4 communities of smokers of each state interacting on a Facebook page. 2400 Facebook publications, which triggered 67000 comments, stimulated interaction. After a 3-week preparation providing information and advice, all participants quit smoking together on March 21, 2017. Then, they received mutual support from peers and professional help daily during 6 months. Professional support included 3 levels: community managers watching and regulating the activity; counsellors providing personalised smoking cessation counselling; physicians providing personalised counselling for complex and medical issues. Personalised counselling at 2^nd and 3^rd levels included help for cessation plans, relapse prevention strategies and use of nicotine replacement. Evaluation surveys were conducted at 1 week, 3 and 6 months among 1270 participants, of whom 37% responded.

Results:
81% of participants attempted to stop smoking. At 6 months,13.5% reported total smoking abstinence while 19% were trying to quit despite occasional consumption. Smoking cessation was not associated with socio-demographic factors. The community dynamics, a strong intention to quit and relapse prevention strategies predicted success while high nicotine dependence, exposure to smokers and negative affect were negative predictors.

Conclusions:
A large-scale smoking cessation program using Facebook, providing mutual and professional support seems effective to reach large numbers of smokers, stimulate smoking cessation attempts and promote smoking abstinence at 6 months.