There's a lot of talk among journalists and political commentators about how blogs are changing politics, but not much hard evidence. We've just posted an academic paper (available at SSRN and here) which presents evidence on who reads which blogs. It analyzes a survey of several thousand Americans, taken in 2006, to describe patterns of blog readership. This data doesn't tell us everything we'd like to know - in particular it doesn't tell us about what is causing people to read blogs, or whether blogs are changing the ways in which they think about politics. However, it does provide us with a map of blog readership. We can tell which blogs seem to be the most popular, which blogs are read more by leftwingers and which by rightwingers, and which kind of people are more likely to engage in politics than others.
First, we can figure out which are the most popular blogs. At least in 2006, we have evidence that the Huffington Post was the most widely read blog in the US. We can also see (graph inserted below) that the most popular blogs have more readers than the least popular ones. Even among the 30 most popular blogs that we gathered data on, there is a steep fall-off between the number of readers for the most popular blog and the number of readers for the 30th most popular.
Second, we can figure out who reads which blogs. As many people have speculated, blog readers are strongly polarized. Left wing readers tend to read left wing blogs, right wing readers to read right wing blogs. Not many people at all read both left wing and right blogs. Those people who do read both left and right wing blogs tend to be much more liberal than conservative, suggesting that lefties pay more attention to rightwing blogs than vice versa. Furthermore, when asked about their ideology, blog readers tend to cluster strongly around the 'strongly liberal' and 'strongly conservative' positions. There aren't many blog readers who are middle of the road. Political philosophers and others who advocate bipartisanship and conversation across party lines are likely to be disappointed by blogs.
Third, we find that blog readers are somewhat more likely than non blog readers to be actively engaged in politics. Even more interestingly, left wing blog readers are more likely than right wing blog readers to participate in politics. Our best-guess explanation is the netroots effect. On the left, you have a vigorous social movement which is encouraging readers to donate to and stump for candidates. On the right, efforts to build a conservative equivalent to the netroots have been less successful. However, we can't prove the existence of a 'netroots effect' with the data that we have. More generally, even if blog readership doesn't go together with polite debate, it does go together with political participation, and this is especially true of left wing blog readership.
This all suggests that blogs have a complicated relationship with American politics. On the one hand, they seem to be associated with polarization in American politics, and perhaps an unwillingness to talk across party lines. On the other, they seem to promote active and direct participation in US politics, something that political observers and political theorists have been worried about for decades. In particular, they may be helping a little bit to organize a resurgent left and help give it a voice.