Do you Tweet? Ever get an e-mail stating that So-and-So is now following you, and you check out their Twitter profile to find zero "Followers" but 20,000 under "Following" (and probably a half-naked woman in the profile pic)? Do you get suspicious? Good call. Twitter is grappling with a solution to these malicious links. (A word of advice: Don't click on the Web site link in those iffy profiles; you could infect your computer, iPhone, etc.)
Here's an excellent Washington Post piece on the growing problem.