WordPress have published an end-of-year wrap-up, which includes some interesting stats. I should think you can find similar figures for Livejournal and Blogger. Notice that although nearly 3 million new WordPress blogs were created in 2008 alone, only just over 3 million WordPress blogs are classed as “active”, meaning they got a human visitor.
2,906,086 blogs were created.
3,761,296 new users joined.
35,926,639 file uploads.
13.5 terabytes of new files.
161.1 terabytes of content transferred from our datacenters.
9,402,615,494 pageviews on WordPress.com, and another 6,467,996,032 on self-hosted blogs (15.8 billion total across all WordPress blogs we track).
3,132,606 active blogs and 44,027,035 active posts where “active” means they got a human visitor.