Concepts such as "porous boundaries" and "low thresholds" appear frequently in the literature on online political engagement. Social media, it is argued, are characterized by less distinct boundaries between non-political and political activities, thereby lowering thresholds into political engagement. This argument is analyzed and empirically tested. Relying on a five-wave panel study among Swedish adolescents, we provide unique insights into the levels and development of political engagement in online political information, interaction, production, and collective action. In sum, the findings show that, for a majority, social interaction in social media coincides with engagement in political information and interaction, while few are engaged in production and collective action. Second, the study provides limited support to the idea that low thresholds in social media promote patterns of tune-in, tune-out political engagement over time. Finally, social interaction in social media has clear effects on online political engagement beyond political socialization and motivation factors.