I think the better question is, who really cares? Everyone is already using Facebook for what Facebook excels at. It's like gasoline-powered cars, now. There are too many people invested in Facebook, and using Facebook, for something to come along and do the exact same thing that Facebook does, but in a different way, and expect people to just move over to it.

A Facebook killer won't come in the form of something similar to Facebook, it'll come in the form of something more time consuming and easier to waste time on than Facebook. It'll be something that lets people do something that they enjoy, and do it every hour or so with a different result. (here, i'm referencing what I think most people do in FB, and that's just browse people's pictures) It won't kill Facebook by being a better Facebook, it'll kill it by attracting people's attention elsewhere.