I would guess it's because the context is relatively obscure whereas with Google it's not. Yes, they can uniquely identify you by your profile more than Google

I would guess it's because the context is relatively obscure whereas with Google it's not. Yes, they can uniquely identify you by your profile more than Google can, but for many who don't 'like' everything left and right (or like everything they come across) how would you determine what ads to show them? Just because my friends like something doesn't mean I will. And that's a wasted impression. Google on the other hand knows what you want - you're actively searching for it. It's pretty straightforward for those who want to serve ads to target what they want to target.