Blockchain is a bad choice for a social network, it's expensive and all data is public. But since we're talking about decentralization, let's make a rough comparison.
Bitcoin: 24229 nodes (source: https://bitnodes.io)
Fediverse: 30005 nodes (source: https://fedilist.com)
Most of blockchain networks are much smaller than Bitcoin, so they don't even come close to the Fediverse in terms of decentralization.