If social media companies will be bifurcated as you say, it’ll be a tiny fraction of their American audiences - leaving the rest of the world sticking with the mainstream. Those smaller Ameri-centric sites will fail to reach critical-mass needed to sustain themselves in the long-term and hopefully those movements will fade into the background.
Your comparisons to the *chan imageboard splintering is apt, but look at how the more extreme chan sites have been having in finding a web-host (with enough capacity) to host them.
Not really. Many other countries also have a left-wing/right wing split in their politics. There is a lot of sharing of thought and cross-influence between right-wing politics and left-wing politics in different countries. Figures on the British right like Lord Monckton [1] or Nigel Farage [2] find many admirers in American and Australian conservatism. Conservative American journalists write puff pieces extolling the virtues of Marion Maréchal [3].
And I think you will similarly find a lot of exchange of ideas and thinkers on the political left between different countries too.
[1] https://www.heartland.org/about-us/who-we-are/lord-christoph...
[2] https://ukipwsx.org/nigel-farages-speech-at-us-heritage-foun...
[3] https://www.theamericanconservative.com/dreher/marion-marech...
After Biden wins, I'd guess we'll have a couple years of laxer enforcement and that will doom the alternatives.
It's not too dissimilar to message board culture from the aughts. You essentially had federated versions of Reddit. It looks like we're destined for that again because people can't stop peeing in the grownups pool.