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I'm hesitant to add yet another meaning to "community" for the same reason. Also because not all instances *are* communities, in any meaningful sense of the word.
What about just "service"? Eg example.social is a service connected to the fediverse. Just like it's a service connected to the net, and presuming it has an https portal, it's a service connected to the web.
I'll admit "service" has a lot of existing meanings too, but most of them apply in this usage.