you’re right that a well-functioning Gab wouldn’t help.
But people don’t have a right to a spot on a platform. And they especially don’t have a right to have a spot on a platform to get their message out. That’s not applicable anywhere else.
ex: I own a private hall and - for arguments sake - I let the local union hold meetings there. And for arguments sake I allow them to do this because I agree with their message and goals. No one else has a right to use the space without my permission, especially if they’re espousing a message I don’t like. Of course it would help those others in getting the message out but I’m not forced to comply.
And it’s even more ridiculous for social media sites. Twitter claims to have 60 million US users. I think it’s lower. It seems to be about 48 million. How many are unique? 35 million? 40 million? The numbers aren’t all that important. But at what threshold does a company lose its property rights?
If you apply that principle consistently, freedom of speech becomes a total joke.
* Hey, the grocery store is a private business, they can deny anyone service!
* Hey, your landlord is a private business, they don't have to let you rent there!
* Hey, banks are private businesses, you have no right to get a loan from them!
* Hey, your employer is a private business, they're under no obligation to employ you!
You can apply this indefinitely. If you do, freedom of speech would become an entity that exists on paper only, like a Cayman Islands shell corporation. You would have the theoretical right, but the moment you exercised it you'd be unemployed, broke, homeless, and hungry.
As for your point about where to draw a line - this is just the sorites paradox. If I run a nightclub, it's totally acceptable to ethnically discriminate, or to tell ugly/fat guests to fuck off. If I run a restaurant, maybe. If I run a fast food joint, no. If I run a grocery store, absolutely not.
Does the same argument apply to my private hall? If 20% of the town has used it does that mean I can no longer decide who gets a platform there? On top of it all I’m now also legally liable for anything those people say or do while on the premises? It would be wiser for me to close it. After all, it’s not really mine at that point anyway.
It would be one or the other. You either run it as a civilized, regulated place, where you can tell people to fuck off for any reason or no reason at all. In that case, you implicitly support those you let in, and are responsible for what they do. Or you run it as a public square, and just let anyone in for any reason.
What you shouldn't be able to do is run something which in every way, shape, and form resembles a public square, but actually isn't. If you could, the 1st amendment would be rendered totally toothless.