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 download the whole social platform in their hard drive or they have to use a third party. It's that simple.
Exactly this.
It seems unrealistic and not to mention expensive. The miners need a good incentive to keep mining and the amount of data being downloaded by everyone would be astronomical! So I doubt anyone could make a decentralised Twitter in this form without demanding some kind of fee, at least in the very beginning.
Perhaps the idea might become more do-able if you cross out the options of posting pictures or anything that needs a lot of storage space. Maybe limiting it to 80-100 Characters as some kind of simplistic Twitter version?
But then you still have the issue of incentivizing the miners.