That becomes more problematic and debatable once infrastructure companies such as a DNS provider or a hosting company refuses to deal with some content given that you remove the ability to create your own soapbox.
My view is that it is functionally a public square, regardless of who owns it. Compare a protest on the side of a road in a company town.
A right to access to an audience, not a right to an audience.
The owner of an auditorium or hall is allowed to not give platform to anyone. They get to decide who gets heard.