© Valentin Flaudias, Ingrid de Chazeron, Oulmann Zerhouni, Jordane Boudesseul, Laurent Begue, Renaud Bouthier, Christel Lévrier, Pierre Michel Llorca, Georges Brousse. Background: Prevention strategies to reduce alcohol use/consumption among young people are crucial to reducing alcohol-related deaths and preventing disease. This paper focuses on the effectiveness of a social networking site (SNS) alcohol prevention program targeted toward young people. Objective: We hypothesized that the program would diminish the relation made by participants between alcohol and festive moments, and would result in a reduction of their declared consumption of alcohol at festive moments during the program. We also explored the interaction with the prevention program that was the most efficient. Methods: The prevention program took the form of 3 lotteries over 2 years. The participants periodically received prevention messages, particularly on alcohol and festive moments (eg, videos on Facebook and short message service [SMS] text messages on their mobile phones). For the 3 periods, the participants had to answer questions exploring the level of their belief that alcohol consumption and festive moments are highly associated. A control group that did not participate in the prevention program was asked the same questions over the same number of days for the first 2 periods. During the second period, the participants were asked to answer questions about their alcohol consumption during parties. During the third period, we explored the interaction with the prevention program on the reduction of their belief that alcohol consumption and festive moments are associated. Results: A total of 651 participants (age: mean 22.24, SD 4.10 years; women: n=430) during the first period, 301 participants (age: mean 21.27, SD 3.07 years; women n=199) during the second period, and 305 (age: mean 22.41, SD 4.65 years; women: n=190) during the third period correctly completed the survey. For the control group, 69 students completed the survey during the first period (age: mean 18.93, SD 1.14 years; women: n=59) and 50 during the second (age: mean 20.78, SD 1.94 years; women: n=45). We observed a significant reduction in the association of alcohol with festive moments in the participants over the 2 years (period 1: z=-4.80, P<.001; period 2: z=-2.11, P=.04; period 3: z=-2.30; P=.02), but not in the controls. We also observed a reduction in the number of glasses consumed during festive moments for the participants (z=-2.36, P=.02), but not for the controls during the second period. The third period showed that only the number of days since registration in the program had an impact on the reduction of the association of festive moments and alcohol consumption (t21=3.186, P=.005). Conclusions: The findings of this study suggest that the SNS prevention program is promising in preventing the association of alcohol with festive moments and, more generally, in impacting social norms.