Over the last decade, an increasing number of academic studies have examined how digital technologies can contribute to political participation, with numerous publications focusing on social networking websites. This article adds to this strand of research by tackling the scarcity of cross-national comparative studies in the field. Drawing from an original dataset acquired by combining a cross-national comparative approach and a mixedmethods methodology, this paper explores how media and political landscapes mediate the contributions of Facebook to citizens' political participation in Italy and the United Kingdom. A participatory gap between Italian and British participants, with Italians displaying higher levels of political participation through Facebook, is found and explained with reference to three contextual factors: the greater diffusion and relevance of other online platforms such as Twitter in the UK; Italian participants' more negative perception of traditional media linked to the high level of political parallelism typical of the Italian media system; and the presence in Italy of a political party such as the Five Stars Movement making full use of the communicative and organizational affordances of Facebook. The findings indicate that the contributions of Facebook, and digital technologies in general, to political participation must be analysed in context, within the larger patterns they fit into, and cannot be examined in isolation. Such contributions are better understood if considered within the hybrid media system in which different digital platforms interact, merge and compete. Similarly, the political scenarios in which citizens and political parties operate need to be accounted for when looking at the links between the Internet and politics.