I missed this from biz school prof Kevin Dooley back in August, 2008:

[H]ow are political blogs the same or different from MSM? Through Wonkosphere, we have noticed that political blogs are consumed in much the same manner as mainstream media is, which indicates that readers treat political blogs not as separate from, but rather as part of, mainstream media. Wonkosphere traffic is greatest on Monday, and tends to peak before breakfast, lunch and dinner, i.e. when people are cruising on the net to end a portion of their work day. Blogs act as newspapers for most.

Second, very few blogs break stories. From our data, the vast majority of bloggers still rely on mainstream media for the content they comment on. In fact, a blogger is just as likely to cite mainstream media as they are another blogger. Thus, bloggers are primarily amplifiers rather than sources of news.

Third, the popularity of political blogs tends to follow a Pareto (power) law, meaning that there are a few blogs that have hundreds of thousands of readers while most blogs only have a handful of readers. This means that the influence of blogs is distributed in the same way, leading to the development of elite blogs (MyDD, Townhall), in the same way we have elite mainstream media sources (New York Times, Newsweek).

Put together, these patterns imply that political blogs are acting as supplements to mainstream media, rather than substitutes for it. Their impact on the system is to increase volatility: blogs make most news spread faster, but sometimes they slow it down. Blogs spread fact, opinion, truth, and slander more rapidly — it is not biased in that regard. Only a few blogs influence opinion most of the time, but any single blog has the potential to impact everyone.