If Facebook's recent privacy issues are any indication, a world of people out there don't understand how this Internet thing works beyond the dimensions of their monitor. Sure, they get the fact that viruses and malware are bad, and identity theft is happening all over the place. But they don't comprehend how deeply their Internet usage is tracked and the level of detail they make available on a constant basis -- not a clue. And somehow, most of them don't care.
Think for a second of what Facebook knows about you. I can't say this for certain, but I think it's safe to assume that Facebook keeps everything you've ever typed into the site, whether or not you've subsequently deleted that information. If it was stored once, assume it's stored forever, even if it's no longer visible to you. This extends to photos you've untagged and anything else tied to your account.
In addition, assume all your Facebook chats are saved and archived. Your IP address is stored for every action and every time you log in. If you're using a mobile app, your exact location may be logged, and if not, your IP address is used to locate you geographically. Cross-referencing all that information gives Facebook a surprisingly clear picture of exactly who you are. Facebook can probably build a very complete profile of users who fail to enter anything other than name, rank, and serial number; the usage patterns are enough.
Talk about manna from heaven for advertisers. To be able to say that the current viewer of a page is a 25-year-old blonde woman who lives with male and female roommates in San Jose, has free time in the evenings, and likes reggae music -- it's superb info for ad targeting. (Wait, how do they know she has roommates? They all log into Facebook from the same IP address and they're on each other's friend lists.)
It's not just Facebook ads that can use that information. Any site that's sufficiently integrated with Facebook laps it up as well, assuming the cookies are in the right place. Furthermore, the fact that you're visiting that site is noted. Facebook can track you all over the Web. This should worry Google tremendously. What Facebook has cooked up has the potential to unseat Google's ad empire, and soon.