I'd argue it doesn't matter. If Taco Bell kicks you out of their store for breaking the rules, you can't sue them because the taco stand down the road isn't a "valid alternative". There were rules of engagement, and you broke them. Since you are the person who violated the contract, you're objectively the guilty party: the extent to which Facebook can punish you is only limited by how invested you are in the platform.
If the point of speaking in public is to be heard, speaking in front of no audience isn't really speaking.
I'm not arguing you're entitle to an audience, either, just that taking your speech from a major player to a much smaller niche player is not nearly equivalent.
The cost of going to a website other than Facebook is literally zero. People do it all the time. The only impediment is the audience's own will and awareness. And they're still free to publicize non-Facebook websites on Facebook. If someone's off-Facebook site flops, that's their fault. Tons of non-Facebook communities have vibrant conversations between multiple speakers and audiences. This site is one.
Maybe I'm just dense, but I'm not seeing the cost differential, either for the speaker or audience. What Facebook is doing is equivalent to saying "you don't have to go home, but you can't stay here." What's so wrong about that?