This research examined how social self-efficacy, collective self-esteem, and need to belong can be used to predict teens’ use of social media. The particular focus was on how these social psychological variables together with social media use account for variation in teens’ participation in a flash mob – an exemplar of 21st-century collective action. Empirical data come from a survey of teens in a major Midwestern city in the USA. Teens’ need to belong was positively associated with the amount of time they reported spending on social networking sites, even when controlling for gender, race, and household socio-economic status. Both teens’ social self-efficacy and time spent on YouTube were positively associated with their intention to participate in a flash mob in the future. These and other findings are discussed in the context of the role of social media in youth culture and collective action.