Facebook is not a public utility. It is not a dot gov. They can allow whatever content they want so long as it doesn't break the law.
If I run a service on an EC2 instance that uses Auth.0, they can shut it off until I agree to use Cognito?
Conversely, should every user of a platform get to dictate terms? No.
In both cases you and a provider enter into a mutually agreeable relation under some terms of service. And you both can terminate that relationship up to terms of service.
We also recognize there's a value in having a diverse set of opinions presented so people can decide on them based on their merits and the best ones can rise to the top. This does mean that sometimes bad opinions are presented, just like sometimes companies that turn out to be bad ideas get started. The solution is not place all authority in the hands of some central dictator who picks the winners and losers beforehand. There is no person or organization capable of reliably filling that role.
I suspect most people agree on this fwiw. But it'd only apply when antitrust laws are invoked successfully - the vast majority of companies and situations do not fall into that category, as viable alternatives exist. E.g. Amazon is most certainly not the only / impossibly-dominant storefront or CDN.
The same goes for OSX and iOS and Android. There are lots of things that can't be run. They're under no obligation to allow them, however much we might want them to.