The problem with Facebook is that it's too big. Different online communities have different standards of what sort of behaviour is acceptable. Facebook is effectively splintered, there is no one community and so there is disagreement on the community standards, to a degree that I don't think can realistically be resolved. Splintering may very well be the result.

If social media were more decentralized, the responsibility would also be decentralized. Standards would set by the communities. And as for overall standards, that would be dealt with by the legislature and courts, which would be a huge improvement, as those are way more transparent and fair than Facebook et al.

Abuse of power by Facebook (or advertisers pressuring them) would be much less of a problem if people could move more easily between social media platforms.

I think a more decentralized model of social media would be good all around. Add some interoperability so you can still communicate when you're not on the same platform, this should alleviate some of the tendencies for these platforms to become so big and centralized.