Facebook has a heck of a lot more info on relationships than an undirected graph with a single edge type. In the front end alone, they now have a directed "follows" relationship. In their backend, they're keeping track of how much you pay attention to people, comment on their posts, click their links, etcetera, and using it, among other things, to decide what shows up in your news feed and what is silently dropped. Someone posted a link here not too long ago describing how to get the weights Facebook ascribes to your social connections, based on the above sort of metric.
Yes, of course they do; they have vast amounts of data; click streams, likes, comment text, etc.
But I guess I'm saying that the core metaphor, with which Facebook did much of its early growth, was that two people are 'friends', or are they are not; and that you see the activity of your friends, and sometimes see activity, and get recommendations, from the friends of your friends.
They built many iterations of the product, and got a very long way, by exposing a very simple social graph metaphor to their users. Even though that model drastically simplifies the world, it has proven to have a lot of utility for users.
Maybe now, because the users are consequently more educated about social networks, a more sophisticated metaphor would go further.
