Antitrust laws were always based on supply side monopolies, while this is a demand side monopoly, and would be a tricky case for anti trust to prove. 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 grap…
Your digital data should be considered to be equivalent to you having built a house, a big asset, but now you have a big choice to make: abandon it because the state is doing some sketchy things that you don't agree with or want to support by the taxes collected, or not.
The lack of data portability, mobility of users, is the only reason Facebook and its leadership could repeatedly be a bad actor and continue to scale as it did; by data portability I also mean that all data should also maintain cross-linking attribution, so if anyone in your network also leaves Facebook to join platform X instead - that data can be imported and aligned, synchronized properly on the other platform so it doesn't get broken. None of the incumbents are going to voluntarily to implement this.