Decentralized architecture is a pretty good middle-ground between centralized and distributed, though (see). Moving to a fully distributed social media -- which would look something like everyone running their own servers -- would carry costs and problems of its own, one of which is very few people have the time and inclination to learn how to do that and massive duplication of effort (everyone becomes responsible for creating and storing their own archives for posterity's sake, which means lots and lots of data will just go to the bit bucket to die)

The data being shared across federated servers allows people to set up 3rd-party archives, which is beneficial, without needlessly burdening instance operators with archival work (sort of a problem for sites like MySpace, there's nothing in it for them except maybe good PR, except digital archiving for posterity is such a niche interest there would likely be little PR benefit to doing so)