According to the Census & Statistics Department, there are still 659,300 daily smokers (11.1%) in Hong Kong. Smoking is addictive, and it is difficult for some motivated smokers to quit without assistance. About 33.2% daily smokers in Hong Kong had tried quitting but they could not maintain abstinence. Relapse prevention is aimed at providing interventions to prevent relapse after starting the abstinence status. This pilot single-blind randomized controlled trial (proof of concept study) will test the effectiveness of social networking services for relapse prevention. The study will first recruit 6 quitters and 6 relapsers for a qualitative interview for the design of intervention. Then, the pilot-RCT trial will recruit 120 participants who have recently quitted smoking, and they will be randomly allocated to 2 intervention groups and one control group. The 2 intervention groups will be arranged to join a WhatsApp or Facebook group for receiving quitting advices from the group moderators and sharing quitting experience with other participants. All participants will receive a self-help booklet of relapse prevention, and will be followed up at 6 weeks, 3 months and 6 months. The primary outcome is the self-report relapse rate (Relapse is tentatively defined as smoking 5 cigarettes in 3 consecutive days since joining the RCT among the 3 study arms.