If the mandate is to do what is best for customers, then the best solution I can think of is to let FB function as one entity but toughen the norms on data collection. The other thing is Facebook should not inhibit any other social network trying to grow using FB's graph. A good example is how Instagram grew with the import of Twitter social graph, and when it came to Facebook, it grew further because it could import the connections that Facebook already had access to. In the name of privacy, it is a good idea to block that, but that essentially inhibits any other social network growing and becoming big. If such an export is available and allowed (while satisfying the privacy concerns), this could enable competition in the social space. As a gatekeeper, Facebook has every intention not to allow any usage to the graph.
The idea about breaking up of social networks would not do any good. You would eventually have to solve multiple problems (and again and again) as the broken up entities either die or become dominant in a big enough niche. Even if you break up Instagram or Whatsapp from Facebook, they are still monopolies in their domains. Insta might not be able to survive since they used the Facebook's ad delivery intelligence, while Whatsapp would be a dominant monopoly in messaging nonetheless.
This is a landmark case if brought on, but needs to be dealt very smartly, if they want to actually solve problems and set the right precedents.