Are there limits to free speech on social media?

Could they even be enforced? 

For my HCC GOVT 2305 class to ponder. 

Some help: 

- Congressional Research Service: Free Speech and the Regulation of Social Media Content.
- Talks on Law: Does the First Amendment Apply to Social Media Companies?
- MIT Technology Review: Users, not tech executives, should decide what constitutes free speech online.
- ABA Journal: Free speech or censorship? Social media litigation is a hot legal battleground.
- Wired: The Parler Bans Open a New Front in the 'Free Speech' Wars.