The article aims to provide a profound understanding of the multifaceted role of adolescents’ social media use in the context of drinking onset. It differentiates between exposure and sharing effects from social media content on drinking behavior and, conversely, exposure to and sharing of alcohol-related content due to drinking in the initiation phase of regular alcohol consumption. We tested our hypotheses based on a two-wave survey among adolescents aged 13 to 17 and focused on those who had not yet been regular drinkers at the first wave of data collection (n = 406). Based on a cross-lagged panel model, we found that exposure to alcohol-related content (exposure effect) as well as the sharing of such content (sharing effect) affected drinking behavior, and that drinking behavior resulted in the sharing of alcohol-related content on social media (selective sharing). We discuss a self-concept verification spiral at the individual level and a social influence spiral at the social level to assess possible risk-reinforcing dynamics.