Blockchain is a decentralized distributed-consensus protocol that uses proof-of-{work/stake} to prevent a Sybil attack (>50% malicious nodes) from taking control of the consensus algorithm.

To recap, blockchain gives you:

1. decentralized
2. distributed consensus
3. with Sybil attack resistance

If you don’t need any one of those 3 attributes then you do not need blockchain.

For the Fediverse, only property 1 (decentralized) is required/desirable. 2 (distributed consensus) is not required since the architecture is Federated; that is, each server decides on it’s own set of available posts, comments and moderator actions, and chooses which of those things to show to users and to federate to other servers. Since property 2 is made redundant by federation, there is no risk of Sybil attack, so property 3 is also redundant.

If you actually wanted to have a decentralized distributed consensus algorithm to ensure that all posts/comments/etc. were replicated exactly across all servers then you could use a distributed consensus algorithm like Raft or Paxos. If you then also wanted to mitigate Sybil attacks on the network then you could simply use a whitelist of trusted peers, as each Fediverse server already uses for federation.

Only if you wanted a truly peer-to-peer decentralized distributed consensus protocol that is resistant to attack would you need to use a blockchain protocol. But, ask yourself why anyone would need such a consensus model when it would preclude localized moderation, “defederation” would result in hard forks of the ledger, and every action in the network would be publically visible for all time making post deletion (moderation) impossible.

Blockchain was invented for online currency where you actually need distributed consensus of a ledger in a Byzantine environment. It’s not really a model that is suitable for social media, messaging, publishing or any of the other applications that it has been shoe-horned into.