From here:
Facebook speaks of the goodness of always sharing with your friends, but it’s really about sharing with people who aren’t your friends, so that marketers can build profiles and buy targeted ads. They’re not bashful about it – it’s in their roadshow video.
Facebook speaks about sharing with your friends on their website. I’m sure they don’t give a shit about skyping because they’re not trafficking in that. Or writing letters to your friends.
It’s a social contract for our times. We will build this site for you to gather with your friends and in exchange we’re gonna make you a fish in the barrel for large corporate interests.