Drawing from the results of a mixed-methods cross-national study focusing on Italy and the United Kingdom, this article explores how the contribution of Facebook to citizens' political participation varies in relation to pre-existing levels and different dimensions of political activity -namely, political expression and information vs political mobilization.The findings indicate that politically active individuals are the ones who take more advantage of the mobilization affordances of Facebook, whereas less politically active participants employ this social networking site mainly for political information. Activists consider Facebook as a key tool for the organization of political initiatives, enabling them to quickly communicate and coordinate, and to operate independently from traditional political institutions such as parties and trade unions. With regard to citizens who engage to a lesser degree in offline and online political activities, the informative power of Facebook 2 and its ease of use come into play. Facebook can, in fact, lower the thresholds of participation by making it more flexible. In addition, political information can reach less engaged users through the activity of participation intermediaries, activating a virtuous circle and potentially producing, in the long run, a mobilization effect.