I don't think you need to. With cryptocurrencies there's a need to obtain global consensus regarding where a coin is transferred from and to in order to prevent double spend. With communications you only need the ability to cryptographically verify the authenticity of the data and who published it. There's no double spend problem.
Looking at email, there is an issue with people tending towards a few large providers and with some providers refusing to federate with other ones for various reasons. The former probably isn't possible to solve directly via technical means. The latter is addressed by the described "self-authenticating" aspect. Peer to peer data transfer coupled with cryptographically based identities means an identity isn't tied to a particular provider.
Who would host the data? BitTorrent protocol has a notorious “seeder/leecher” problem. And how would that hosting mechanism be resistant to collusion and manipulation?
For example, a signature or zero knowledge proof is published onto a social network by the president’s cryptographic identity, how can we (the people viewing the network) verify that the data we see on the network is in fact published by the president, and not a malicious actor who is hosting their own node? At a scale of just one individual, we could easily establish “Person A = public key hash X” and verify that, but at a scale of millions of users it seems you will need some sort of decentralized state/data tracking (like a distributed blockchain ledger).
Anyone who wanted to. If you wanted to make sure your personal data remained available when you turned your computer off or that it could be downloaded quickly then you'd need to arrange for paid hosting. Pick up cheap vps and share it with your family and friends. Whatever.
> how would that hosting mechanism be resistant to collusion and manipulation
It doesn't need to be, that's the point of using strong cryptography.
> it seems you will need some sort of decentralized state/data tracking (like a distributed blockchain ledger)
No, you just need to keep track of keys. Your identity is your key. We already do this. Certificate authorities, ssh keys, etc. The issue you're describing is one of using a centralized authority to solve the problem which is hardly the only solution and certainly doesn't require a blockchain.
Presumably Bluesky's social media protocol will need to operate on some data/ledger structure, to maintain a large social graph (follower/follows), user records (public key hashes, posts, etc). If too few nodes are hosting this data, it would be malleable to manipulation by the hosts.
The same occurs in blockchains that are not widely distributed enough: a blockchain with 5 validators (hosts) is more susceptible to manipulation (i.e. hosts changing records/data for their own gain) than a blockchain with 50K validators, because the cost to control the network becomes exponentially expensive. Strong cryptography has nothing to do with this, and does not prevent this kind of manipulation. Imagine a sort of 51% attack on Bluesky's social graph, in order to manipulate a user's associated public key hash or alter the social graph for some nefarious reason.
A blockchain ledger probably is not the only solution, but it does create a strong financial barrier to manipulation (i.e. 51% attacking a widely-distributed PoS blockchain ledger would be prohibitively expensive and short-lived). I am interested if Bluesky is considering these problems and whether they have alternative non-crypto/blockchain solutions.
Looking at email, there is an issue with people tending towards a few large providers and with some providers refusing to federate with other ones for various reasons. The former probably isn't possible to solve directly via technical means. The latter is addressed by the described "self-authenticating" aspect. Peer to peer data transfer coupled with cryptographically based identities means an identity isn't tied to a particular provider.