According to the latest (January 2005) Pew Internet & American Life project report, 7 % of the Americans say they have created a blog or a web-based diary. In springtime 2002, the figure was 3 %. Furthermore, 57 % of these more than 8 million bloggers are male. This surprises me as according to the figures I've seen (here and here), the majority of bloggers are women. Perhaps this is due to the fact that the Pew Internet report only looks at adult Americans, and therefore overlooking the numerous teenage female bloggers.
Another interesting finding is that though blog readership increases (growth in 2004 alone: 58 %), not many know what a blog is (62 % don't).