When you use Facebook, you're always logged in, and your identity and relationships - to others, to content, to apps and services - are assets Facebook can use to customize your experience (oh, and your ads).

Anytime Facebook wants to change how it might use all that data about you, in any way, across any service it has within the Facebook ecosystem, all it has to do is change one privacy policy, tell you about it, and that's that.

You happily give Facebook terabytes of structured data about yourself, content with the implicit tradeoff that Facebook is going to give you a social service that makes your life better.

John Battelle Quotes