You say this like it's some weird, bizarre thing. Who else should be judge and jury by default on a private platform? Should there be some government committee any time a new social media company starts up?

In any case, to answer your question: if the issue is that they've become too powerful, and there's a reasonable case to make there, then I'd say the answer is to either break them up or support alternatives, not have the government barge in and micromanage how they run the particulars of their business. I don't think there's some obvious right answer for "how tightly should a social networking company restrict what people say", so the best possible answer from a societal level is to help choices flourish and let consumers choose.

Every platform has some 'censorship' -- even Parler banned porn, and got rid of left wing 'trolls'. The only real way around it is to not have a platform so much as a protocol, like email.