Let's think of your personal information as your orifice of choice. Your likes, dislikes, the things you share on Facebook, all that stuff combined is your orifice. You could be selling it yourself, but since you're not sure how to market it and profit from it, you don't.
A corporation is a John and the ads are its penis
Corporations want to use your orifice so they can deliver their product. They want to use your personal information to get their product inside.
Facebook is your Pimp
Facebook let's corporations get the benefit of your orifice, but control their access to it and take the profits from it's use.
Bad analogy, or close enough?