The Center for Media Research has caught up to some BIGResearch data from February on who is blogging, and while the data is getting a little stale (as these things do about every week or two), its analysis is worth examining.
They're younger, more Democrat than Republican, more African-American and Hispanic than not, and more often male than female. They're a little lower-incomed and higher-educated. Even though they're content creators, they lean on articles and other sources for their posts.
What's more to the point is that bloggers seem to be everyone. The data suggests some 26 per cent of adults say they blog. Well, that's a figure I'd be a little suspicious about, but at even half that level, it's a general function and no longer a niche. The center's analysis is here.