Generally, I think the law in the US is that a good faith attempt must at least be made to keep stuff like extremist speech and illegal content off websites. Big site owners can’t wink and nod about it (or shout loudly like they do on Twitter), and they can’t secretly refuse to enforce rules to maximize profits (like they do on Facebook etc).

And by “can’t” I mean “shouldn’t,” because I strongly doubt these companies will be held responsible for much.