"The advertisers are the users, and you are the product." ~Don Norman
Don said it about Google, but it's actually true of any service that sells ads: Facebook, Twitter, television, ...
Source: http://gigaom.com/2011/09/05/don-norman-google-doesnt-get-people-it-sells-them/
It's particularly interesting to think of this in the context of Facebook's profound changes: http://mashable.com/2011/09/21/prepare-for-the-new-facebook/
Facebook's changes have are about people connecting more emotionally with the Facebook service.
Emotional valence -- and the resulting bonding -- leads to better product.
Lifehacker repeats the point: If you're not paying for it, you are the product.