IT is working on A LOT of infected machines that picked up their badness from Facebook.  At this point, if we see less than 3 a week, I would be shocked.  Now, in addition to malware and spyware coming from the site, Facebook is gaining attention as a haven for scammers.  Give the article below a read for additional information:

http://www.techcrunch.com/2009/10/31/scamville-the-social-gaming-ecosystem-of-hell/