If you think that is scary, MS was caught by Google sending what search results people were clicking on, and for what key words. (Google figured out MS was copying some of their search results into Bing and ran some tests to verify that they could get Bing to return nonsense results based on Google returning nonsense first.) In theory it's not traceable back to individual users, but.
That said, FB is pretty "open" about how aggressive they can be scanning your address book and every message you've ever sent or received, especially if you link your email account to Facebook. Have you installed any plugins even vaguely related to Facebook? They could be tracking you that way as well. (sandboxie won't help you if you have something sending data back to their servers.)
I've eschewed Facebook so I can't be bothered with getting fact, but I believe they track everything they feel they can legally track in order to build a profile on you so they can sell your eyes to advertisers. After all, when you don't pay for a service it's because you're the product.
