As pooya87 said, there isn't such thing, because of lack of incentive. In block chains, the users verify every single information they receive. How will this work in a “Decentralized Twitter”? Will they download every single message, image, video, podcast etc.? I hope you understand how impractical this is.

Either the users keep the whole social platform in their hard drive or they have to use a third party. It's that simple.