Use of social media by political parties has become a part of their communication strategies. In Catalonia, where around 20% of Internet users obtain political information through Facebook and Twitter, parties use these channels widely. This article has examined 814 posts, 5,772 comments, 52,470 likes and 25,907 shares from the official Facebook pages of Catalan parties in order to ascertain the relevance of the classical party characteristics (party size, level of institutionalization, centralization of decision-making, position at the ideological cleavages) on how parties and their followers behave on Facebook. The data sustain that the characteristics of Catalan parties have an influence on their posting behaviour on Facebook, and mould the reactions (comments, likes and shares) of their Facebook followers to these posts. The results further show that small and new parties achieve greater engagement than bigger and more institutionalized parties.