In a bout of academic navel gazing I looked into available research on political blogs. I found "Self-segregation or deliberation? Blog readership, participation, and polarization in American politics," by Eric Lawrence, John Sides, and Henry Farrell, in Perspectives on Politics March 2010. (The actual article is behind a subscription wall but a pdf of the conference paper it is based on is at http://www.themonkeycage.org/blogpaper.pdf). The data is a little old, based on a 2006 survey but the conclusions are interesting. Here is a quick excerpt:
Readers of blogs of difference ideological dispositions do not participate less than those who read only blogs of one ideological disposition. Instead, readers of both left- and right-wing blogs and readers of exclusively leftwing blogs participate at similar levels, and both participate more than readers of exclusively right-wing blogs. This may reflect social movement-building efforts by left-wing bloggers.
It's interesting -- exposure to points of view different from you own make you more tolerant but less politically active.