There have traditionally been two types of monopoly, exemplified by Standard Oil and AT&T. Standard Oil came about through mergers. This is the one where antitrust fits -- you break them up, problem solved. AT&T is a natural monopoly. Breaking them up doesn't fix it; the last mile has to be treated as a public utility because a local monopoly is still a monopoly and it's not practical to build fifteen independent last mile networks everywhere.

But Facebook, despite being a monopoly or nearly so, isn't either of these. It makes no sense to speak of breaking Facebook up -- what does that even look like? Each if the Baby Books gets 1/20th of the users?

But it's also not a natural monopoly because what makes Facebook a monopoly is a lack of interoperability. If "Facebook" was a federated system where anyone can set up their own social networking server and still communicate with anyone else on any provider, there would be no monopoly. Some people would use Google and some people would use Microsoft and some people would have their own server in their basement and they could all still talk to each other. Social networking is not a natural monopoly any more than email is -- unless you allow the largest player to refuse interoperability with competitors.