I think the more salient question is "why do people post everything on facebook (and other services)". No sane person can be surprised that this is what happens with your data that you freely give away. And what I have been hearing nobody actually is surprised. Nor do they care in the end. Selling adverts is just an extremely lucrative business, get over it. It is what pays for your entertainment (sports etc), information (magazines, internet sites etc) and just about everything.