The first step to a Yelp killer? Type in "Italian Restaurant" and get back list of all the restaurants your friends have checked into. A recommendation engine.
For now, its mostly to filter your facebook data. But do you think it can expand and become the goto search engine, ala google?
But I think Facebook overhypes how much people care about their friend's preferences, at least for most things. We generally don't pick friends based on how closely their shopping/eating/whatever habits match ours.
It seems like Facebook and other social networks took the idea of asking your friends for a recommendation on a good doctor or auto shop and twisted it into some overly simplified premise that if one friend likes something, all of my friends will like it too.