Another option is that the sending server should send the public activity to every *server* on the network, instead. So, if there are on average N actors per server on the network, it's (all users)/N, which might be tractable, although it's still pretty big on the modern Fediverse (~30,000 servers, according to fedidb.org). But, again, there's not a big list of all the available servers that the sending server can use for publishing to.
For ActivityPub, we didn't say that public activities should be sent to every server. Instead, we said that they should be sent to every server that the sending server knew about -- usually because an actor on the sending server had interacted with an actor on the receiving server.
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