This doesn't matter from the standard of antitrust law. What most fail to grasp is that being a monopoly is not illegal. It's only illegal to use monopoly power to harm consumer welfare.
In this case integration between services is a pretty strong defense against this. Facebook can argue that consolidating the infrastructure allows them to deliver each product at a lower cost and/or higher quality than if it was served independently.
If the court accepted those facts, then Facebook would have a ironclad argument against being broken up under the consumer welfare standard.