Here's my take on Facebook (disclaimer: I work for Google, these opinions are purely my own and not representative of the company, yadda yadda yadda). Display advertising works best when you have sufficient, quality inventory (publishers whose sites serve ads) such that you attract advertisers in sufficient quantity and quality. Targeting in display ads works because from the publishers, the intermediary has informat…
1. Figure out when people want something and what they want. And insert suggestive ads in people's streams in a way that is not creepy. Or gamble that beyond a token uproar, they will not disturb their equilibrium enough to allow a competitor's toy to slowly siphon their users.
Facebook could also figure out when people are looking to give gifts and aid in that arduous process with targeted coupon/ads.
2. Lobby to become the premier centralized digital consolidated identity provider once governments start to require this due to some misapprehension of how much security such a requirement would bring. With this toehold, Facebook would have more room to control your stream and could insert creepily timely targetted ads. The facebooks of the future may become the synthesis of insurance and big media companies of today.