Yeah I'm not even sure how you would break up facebook, other than facebook/instagram . It would make you extremely unpopular if they forced FB to break up it's userbase into thirds or something as they'd all just flock to one third and it would just be the same thing again within a few months
I think a more effective solution than a breakup would be requiring Facebook to interoperate with other social networks. We could force them to implement APIs which would let users from other platforms chat with and send posts to Facebook users. Imagine a tweet or a snap showing up in your Facebook feed, and vice versa. ActivityPub[0] is already a protocol which tries to solve this, but Facebook would never freely implement it because it would breach their walled garden. We might be able to regulate them into doing it.
As other suggested, forcing interoperability would go a long way (it should be noted that e.g. facebook messenger WAS interoperable when Facebook wasn't as large, but they stopped it when they became the leader).
Additionally, separate to instagram / facebook / whatsapp ; possibly also facebook social / facebook marketplace / facebook events ; They should have a level of access that others have. Imagine Yahoo users could subscribe to Facebook events without ever setting foot in facebook.com ... I know it sounds like science fiction, but it's possible. like email....
