By tricking users into making more information public they get more hits and get more opportunities to 'share' your data with 3rd parties.
What is your explanation for why facebook removed the ability to make your profile picture private to non-friends?
Basically, if your profile picture were private, it would be hard for someone (who is your friend/acquaintance in real life) who isn't friends with you (on Facebook) already to find you on the site.
That is very different from this: "The only way they can do it is by selling users data one way or another.". People often claim that Facebook wants to "sell" your data to advertisers, and I'm simply saying that claim is wrong.
(I've never been secretive about the fact that I worked at Facebook, it's on my profile. )
Now, you can argue that making private information public to generate revenues is not selling, however, IMO, it is, given the fact that they started out as a completely private social network.