Facebook gives your info to advertisers, gets caught in the act, promises to stop
Facebook was giving away enough info that advertisers could identify your individual account and see which ads you'd clicked on. The companies receiving this data included major ad networks like Google-owned DoubleClick and Yahoo!-owned Right Media.
I write this all in the past tense, because Facebook has agreed to stop leaking this data in the wake of the Journal's report. As for the data that's already leaked, DoubleClick and Right Media have both said they didn't know they could access the data in question, and hadn't been using it.
Part of the problem is that Facebook and MySpace don't do anything to obscure your name or ID number in your profile URL, so that information is passed through to advertisers when you click. Closing that loophole seems like a simple (and important) security fix.