Facebook's Like button was really a brilliant move by Zuckerberg because it essentially gave FB a tracking page of the majority of the Internet. I don't mean this in a nefarious way: this is how display advertising works. A cookie allows the advertiser to see what sites you visit and, from your behaviour, develop a profile of your interests, likely age/gender, etc (all based on modelling).
I should also point out that before the inevitable cries about privacy, an advertiser doesn't care about you, specifically. They care about the characteristics of groups of people because that's the only way statistical modelling works anyway. If you visit some fishing sites, you as an individual may or may not be interested in fishing. Take 1000 people like you and there's a much higher chance that a member of that group is much more interested in fishing than the general population.
Anyway, I thought a couple of years ago that FB was poised to become a major player in the display advertising business. This hasn't come to pass. Not because they couldn't. They simply have chosen not to go down that path. They don't' want to risk cheapening their platform by becoming another AdSense (which is the lowest CPM you can get and is really for the very long tail).
This is one reason advertising is tricky. It's a balancing act where you're trying to cultivate premium publishers and advertisers while segmenting both (premium, remnant, long tail, etc) without cheapening the premium side, which is where you make all your money.
So you can buy an impression on the WSJ through a guaranteed reservation. This by far generates the most money for the site and is how the WSJ would like you to buy advertising. Still, they can't sell all their impressions this way so there are various sources for monetizing so-called "remnant inventory". Thing is, you don't want your premium advertisers buying impressions through those (cheaper) channels instead.
So Facebook seems to clearly be protecting its potential value. It really is potential value until it's realized.
I have a theory about this: I don't think all that data we give Facebook is actually all that valuable. What sites you visit on the Internet vs what you tell Facebook is really the difference between what you do and what you say. What you say is filtered through how you perveive yourself, how you think others perceive you and how you want them to perceive you. What you do (IMHO) is a far more genuine representation.
So time will tell. I think at this point if you see FB enter the display advertising space, it's a de facto admission of failure (meaning it hasn't met expectations) of monetizing all that data.
Disclaimer: I work on display advertising for Google.