(DO NOT visit the following URLs. They're for reference only.)
With the recent MySpace exploit that went around, I would have figured the social networking scene would be really sensitive to this. However, that is apparently not the case.
One of Facebook's advertisers is pushing a spyware trojan horse onto their users.
If that fails, as it will in a web browser that isn't Internet Explorer, a variety of other fail-safe conditions occur. Honestly, I don't have the time of patience to explore them. The ending is, essentially, they'll try to upload http://188.8.131.52/jk/exp.wmf to your machine. This WMF file uses an "old" exploit to force an install of http://www3.adoptium.com/jk/loader.exe on to your computer.
And then you're fucked with spyware you can't be rid of.
UPDATE: They accepted advertising in http://ads.facebook.com/ads/247/728x90_fb.html from RealMedia. RealMedia hooks up with PrecisionClick. Twisty passages gets you to Adoptium.
It's not all Facebook's fault. They just wanted some money, and the advertisers of ill-repute apparently pay the best. What a shock.