Click Fraud and Spam Blogs
This passage was particularly illuminating in explaining the recent growth in spam blogs:
"When you connect the dots, what seems to be happening is that scammers are doing four things. First, they create a lot of fake blogs. There are slimy companies that make easy to use software to do this for you. They scrape bits and pieces of legitimate blogs and repost them, as if they were just another link blog. It is very hard to tell the difference between a fake blog and a real blog until you read it for a while and realize there's no human brain behind it, like one of those Jack Format radio stations that fired all their DJs, or maybe FEMA. Then, they sign up for AdSense. Then you buy or rent a network of zombie PCs (that is, home computers that are attached to the Internet permanently which have been infected by a virus allowing them to be controlled remotely). Finally, use those zombie PCs to simulate clicks on the links on your blog. Because the zombie PCs are all over the Internet, they appear to be legit links coming from all over the Internet."