This paper fills in a gap in the academic literature by paying attention to the fragmentation effects of social media. Preserving a common public agenda is important for social integration, for minimizing social cleavages and reducing polarization. Although observers have warned of the risks of social media for audience fragmentation, research has not devoted much attention to the role of these platforms of news distribution in preserving the common agenda. We contribute to this literature by measuring the effects of social media over the public agenda at the level of public cognitions. A second contribution of this study is that we use unobtrusive data, i.e. observed online behaviour, to measure consumption of news. We relate the effects of social media as referral to news, to individuals’ personal agendas. We find that when Facebook is a relevant news referral, it reduces the amount of agreement about a common public agenda. Our results also suggest that different social media platforms might have varying effects on the public agenda.