With millions of daily users Mainline DHT is the most successful truly decentralized social network.
Successful decentralization is about incentives, and Mainline DHT's incentive is downloading digital media for free.
I think the blogosphere is the most succesful distributed social network. People just dont like viewing it that way.
it's only the storage infra, though. but it stores content, nodes, and messages in the same DHT.