> They should have an obligation not to censor, because between them they can virtually block all practically available channels of communication

Absolute bullshit. It has never been easier in history to publish your own thoughts for the consumption of anyone who is interested in reading them. You can make your own website and put just about whatever you want on it. You can write and publish pamphlets or books with print on demand services. You can record audio or video with your phone and put it on your website or just send it directly to people. You can walk down to the town square and say pretty much whatever you want.

You absolutely don't need to be on Facebook or Twitter or ANY social networks to exercise your free speech. None of these companies has power over any means of communication other than their own platforms. You don't have to use their platforms.