Indeed, separating Facebook as is would be close to impossible.

However, a first step would be imposing FB to expose all features and user data using an open protocol to be defined.

That would be a good thing on its own, given that it would make possible for other networks to federate with Facebook. (And when you think about it, it's much closer to the usual Internet, think of SMTP, BGP... A monolithic service like FB is actually the exception!)

Moreover, it would set the ground for splitting Facebook into two (or possibily more) federated networks.