About "closed" social networks, when people prefer to stay in their small world, I heard at #SplinterCon the terms : "dark forest", "cozy Web" or "postviral era".
Interesting discussion about the differences between #Matrix and #ActivityPub at #SplinterCon .
Matrix, unlike ActivityPub, separates the group/room (a social concept) from the server/instance (a technical concept) so you don't choose a server for content or content control reasons, just for technical reasons.
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