They are two different things, though. One is actually producing content, and the others deciding which content host and share. And there are all kinds of various legal and illegal combinations, here. For instance maybe they decide that it's okay to host Nazi content, something that is absolutely protected under the first amendment. Or maybe they decide that it's not okay to host Nazi content, even though it's definitely protected under the first amendment.

Also see Gonzales v. Google.

But really the most dangerous thing here is telling a company that they are legally liable for everything their users post. A large company like Google has the legal firepower to handle the massive onslaught of lawsuits that will instantly occur. A smaller startup thing? Not a chance. They're DOA.

Heck, even on my tiny traffic personal website, I would take the comment section down because there's no way I can handle a lawsuit over something somebody posted there.

I should not be required to host content I do not wish to host. And at the same time I must be shielded from liability from comments that people make on my website, if we are to have a comment section at all.