Facebook did not create the social graph. It merely tried to map what already existed out there. But then a social graph where everyone is a friend is not the best possible map.
There are many social graphs. That is our reality. There will be many maps. Facebook is like a broad map of the world. And it can hope to become better and better, but sometimes you don't want a map of the world, you just want a map of the NYC subway system. Or maybe most of the time you just want that.