@stefan In 2024, the relationship between ActivityPub and BlueSky is like that between OStatus* and Diaspora. In both cases, both are working on open, decentralised protocols. But the latter has gone for a new protocol, to add things they perceived as missing from the older protocol.
This creates a new space for protocol experimentation, but it means people have to have accounts on both to talk to everyone.
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*the first protocol used in the fediverse, before ActivityPub existed