Pew Internet & American Life Project measures the impact of blogs.
8 million American adults say they have created blogs; blog readership jumped 58% in 2004 and now stands at 27% of internet users; 5% of internet users say they use RSS aggregators or XML readers to get the news and other information delivered from blogs and content-rich Web sites as it is posted online; and 12% of internet users have posted comments or other material on blogs. Still, 62% of internet users do not know what a blog is.
It shows just how dramatic the uptake in blogs from both a publishing and subscribing perspective has been recently. I can only see the popularity of this information sharing medium increasing.