Pew Internet & American Life Project released a report called The state of blogging, where they note the following:

• 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
• 12% of internet users have posted comments or other material on blogs
• 62% of internet users do not know what a blog is.

Click here for their report, downloadable in PDF form.