As of this writing, Technorati (see below) is tracking over 70 million blogs. Estimates show that the total number of blogs doubles every six months. It should be noted that this trajectory includes all blogs, including every random posting by millions of MySpace users. With regard to more professional (is that the right term?) blogs, the increase may be more like a doubling every year.
In the past year I've written quite a bit about the concept of the "Long Tail" with regard to Web 2.0 and search engine optimization. There is, of course, a very long tail of blogs. There are a few blogs that make up the head of the blog charts - sometimes referred to as "A-List" blogs. For every niche, there are the most popular A-List bloggers. They have become media voices in their own right, even rivaling their offline competitors for attention of readers and advertisers. But because the bar to entry is so low in the blogging world, there are many voices, and not all achieve superblogger status, but they do all garner some attention and so contribute to online conversations.