This paper examines panel data from two waves of the Youth Participatory Politics Survey, a nationally representative sample of young people in the United States. It employs a cross-lagged design to investigate the extent to which common forms of online activity create pathways to online and offline forms of political activity. Specifically, we examine the influence of Friendship-Driven (FD) and Interest-Driven (ID) online activity on online participatory politics and on offline forms of political action. Our findings reveal that FD and ID activity relate to political engagement, but in different ways. In addition, we find that the size of young people’s social networks interacts with both FD and ID online activity to promote political activity. This indicates that having exposure to “weak-ties” (resulting from large social networks) promote higher levels of political engagement. These findings demonstrate the need to specify the kinds of online activities in which youth are engaged and, more broadly, the political significance of social media and social networks.