I've had my blog now for over six months, and one ofthe things that has surprised me the most is the spam you get via the comments section. Here's a sample of a message I got recently:
"I simply want to tell you that I'm beginner to blogging and site-building and honestly enjoyed you're web page. Probably I’m likely to bookmark your site . You actually have beneficial stories. With thanks for sharing your blog."
You see a message like this and you think, "wow, some guy that is clearly not American has been reading my blog and enjoying what I write. Nice!". Then you google that message and realize he did the same on a million other sites, likely to boost the visibility of the sketchy site he runs. The other popular spam I get a lot is trying to sell pharmaceuticals.
As is true with all spam, the only reason people keep spamming is because a very small percentage of people actually fall for it and buy something (or fall victim by giving their credit card). It sometimes amazes me that people can be so gullable, but I guess I tend to underestimate the stupidity of many people.