This is all new territory, but good public policy imho would be to punish any corporation that sorta looked like a public carrier (whether it technically was one or not) that tried to censor users based on their political views.
Anti-trust looks like the best tool in the tool-kit to get the job done.
(Facebook, Google, Amazon etal are becoming zucking dangerous and this is not a good time to hide behind momma’s skirts of libertarian ideology.)
One can argue that there are "alternatives" to the companies you just named however if we look at the size, scope, influence of Fascistbook, Google, Amazon etc.. they are in effect monopolies.
Google for example has Bing and Yahoo as "competitors." All of Google's "other" competitors that I'm aware of are nothing more than front ends into Google to perform searches.
Yahoo as a search engine is a joke. Bing is still trying to catch up to Google in breadth and depth of search results.
The FedGov has an "interest" in regulating monopolies for a variety of reasons, the biggest of which is spurring competition, allegedly. Given the size, breadth, depth, name recognition, technical superiority (and ability to manipulate results, let's not forget that...) how long would it take someone to stand up a credible competitor to Google? With all of Microsoft's resources and how much they've thrown at Bing over the last ten years, I don't know of anyone who'd call them a credible competitor.
I don't know if anti-trust is the answer (it may be ....) I'd like to see an audit of Google, Fascistbook, Apple, etc.. and examine their financials to see how their market values are being cooked up/manipulated and prosecute any financial shenanigans. I'm a follow the money kinda guy. I'd bet the farm they're manipulating their values as a way to hold off competition.