This paper deals with online political discussion on social networking sites. Drawing from Habermas’ concept of the public sphere and former adaptations of public sphere theory to Internet research, the study examines to what extent political discussion on social networking sites displays public issue focus as well as deliberative, liberal and communitarian characteristics. The empirical analysis is a case study that scrutinizes two opposing Facebook pages created in the context of the topic ‘Stuttgart 21’ – a construction project that evoked a local civic protest movement in the city of Stuttgart in the south of Germany. Using an ethnographic approach, the study takes into account the architecture, culture and discussion style on the two pages and aims at describing the pages in terms of their degree of reciprocity, contestation, ideological homogeneity, rationality and contextualisation with the offline protest movement. The results show two polarized pages that lack deliberation and dialogue, but feature ideological homophily and identification. The results back the fragmentation theory of Internet audiences, while not maintaining the fear of losing the common ground in society. On the contrary, the study suggests that civic political engagement on social networking sites should be discussed in the context of radical democratic processes. It concludes that the utilization of social networks in order to politically inform, stimulate and mobilise scalable publics is desirable.