Facebook is almost like a giant advertising storehouse. If you put down any interests or activities, email or usn, you're basically screwed and will often be targeted as you continue to use the service. If you put down "metal" as your interests, you'll see sidebar advertisements asking you to like the new Nile album or something stupid like that. On top of that, if you do a search for your name on google or something, after you've officially "liked" doom metal, you'll find your name and status updates on a separate, special interest page for doom metal. It makes ad targeting much easier.
...then again, that's also an opportunity for smaller/ideologically based groups to reach people. I'm kind of of torn on it, honestly. After all, people mostly use facebook to contact old high school class mates (the most mastubatory thing ever) or get in touch with friends at a distance. I'm not sure if what happened to myspace (crashing your internet browser with excessive ads and user stupidity) will happen to facebook, but they definitely have their paws all over everyone's interests....strange as that is.