[chadwickthebold]

You just answered your own question. It is generally accepted that Facebook and Google derive a large source of their revenue from advertisement, which is in such high demand at these companies precisely because they control so much of your personal data.

It isn't so much of a leap of reasoning to think that a company willing to sell some information about you isn't willing to go further than you'd like them to. Again, it's not that we're worried about them 'leaking' it - we're worried about how they're going to sell and make money off of it.