> Geographically, like a phone network.

I don't think that makes much sense for breaking up something like Facebook.

Unlike a phone network, Facebook's service is not rooted in geography. Yes, it's users have homes and geographic locations, but those are just arbitrary account attributes, just like their name or if they like the Denver Broncos. You might as well have one post-breakup company start out with all the A-M accounts and the other the N-Z accounts.

If you had to divide up the users between N successor companies, I'd probably be best to do it based on friend-graph connectivity, with mandated interoperability between them. That might make the successors sticky enough to survive.

There's a high correlation between geography and social graph, while the part of the social graph that gets "cut" will give an incentive to make interoperability work, as people will care about it.

So your premise is true(ish), but the conclusion can't be drawn from it (and your comparison falls apart) because we don't need strict dependency on geography, only a (relatively strong) correlation.