There is no way to apply state-actor First Amendment standards to social media platforms that wouldn’t amount to either destroying the platforms or eviscerating the protections of the First Amendment against state action.
If you’re going to conflate Facebook and the federal government, then you have a Facebook, Twitter, YouTube, etc. that can’t ban porn, that can’t ban snuff films, that can’t ban animal torture videos, that can’t ban commercial spam, and that can’t ban any targeted harassment short of credible true threats of violence. It’s not just about the Alt-Lite Podbros screaming their favourite slurs. It’s about applying the entire body of First Amendment jurisprudence in all its sweeping breadth.
Either you wind up with a legal mandate that the entire Internet must be a 4chan dumpster fire that most people wouldn’t want to use, or you water down First Amendment standards to create arbitrary special protections for Alex Jones Pizzagate videos while excluding Ron Jeremy’s greatest hits.
Outsource content moderation decisions for these companies to the government and it really does become censorship. Not some tenuous cultural-norm analogy to censorship, but actual state censorship imposed by government agents and backed up by real force.