You've been drinking the Kool-Aid liability is no-cure all solution, these platforms will merely cease to exist. I agree that they have quasi-monopoly powers, there are other platforms that have this though, they are now basically modern forms of Press & Media. Editorial decisions whether to publish or not and whether to self or government regulate are issues the world has experience with. The difference is in the scale and the speed at which these platforms can disseminate information. There is no clear cut solution (you'll see this if you look into press & media regulation) and society needs to decide how to move forward.