Irrespective of content? What about just flat out defamation? What about CSAM? How about organizing a general strike? What about ISIS propaganda? Or bomb making instructions?
1. People post defamatory stuff on social media sites all the time. That's a civil matter. People can sue the poster over such things, but not the site. See "Zeran v. America Online, Inc." 2. CSAM is illegal, but people have posted it on Twitter, and they're still around. 3. I'd imagine that calling for a general strike is protected by 1A. 4. Terrorist propaganda can be posted by a US citizen, as "incitement to terrorism" is protected by 1A. You just can't provide material assistance to a foreign terrorist organization. 5. There's actually a law on the books supposedly banning this, but I don't see how it's enforceable.