It's ... hard. But consider my idea of people randomly PoWing the people they follow. Even mobile-only people would occasionally get some server PoW. Over time pubkeys would accumulate a lot of it, and new accounts would have just a little. It's not perfect, but at least no one would wonder which @jack is the real one, and maybe some new variant will work even better

I'm not quite following. Would you elaborate? Seems to me, a bottleneck on keypair generation would reduce the number of accounts that can be generated for spam, so it would significantly reduce spam as spammer have to continuously generate new keys as they're blocked or filtered by relays. This of course doesnt take into account message level PoW, where one must be in the hash of the messages themselves, which clients and relays could implement. And none of it addresses how PoW requirements are communicated between parties, both users and relays, to ensure that everything doesnt just become some standard, which is sure to be a difficulty of 0 in that case. I should he able to set a PoW as a user for DMs from users not in my follows list, for example, or something like that. Seems to me it would be better than WoT because it wouldnt shut new keypairs out of the network effects.