27% Of Internet Users Read Blogs, 62% Don't Know What A Blog Is
By the end of 2004 blogs had established themselves as a key part of online culture. Two surveys by the Pew Internet & American Life Project in November established new contours for the blogosphere: 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.
http://www.pewinternet.org/PPF/r/144/report_display.asp
Interesting report, especially if you're interested in the marketing aspects of blogs.