It depends. And the problem that people have is that social media companies want to be both publishers and platforms.
For example, T-Mobile is a platform. They aren't responsible for anything you say when on the phone, using their network.
CNN is a publisher. They are responsible for anything that gets posted on their website, and can get sued accordingly.
Social media companies want to choose what is posted on their website, but also not be held responsible for anything that is posted on their website. They want the perks of being a publisher, and the perks of being a platform.
Obviously there are arguments made on both sides. But that is the general disagreement, if I understand correctly.