Participatory social media platforms facilitate the political communication and cooperation of interested citizens, leading, some believe, to the strengthening of political and civic participation offline. Given such a tangible effect of social media on political engagement, how do politicians react to their proliferation? Previous studies have analyzed antecedents of the adoption of new media applications in online campaigning. However, only little is known about the drivers of social media use in day-to-day political routine, outside of a campaign setting. The present study measures the frequency of political communication by members of the German federal parliament on a variety of social media platforms throughout the year 2010 - a year without any elections at either the federal or state level in Germany. It tests for a range of possible antecedents, and finds that supply-side (age), demand-side (electoral district type), and structural factors (party affiliation) significantly impact the use of social media by parliamentarians.