(Finally, something comes up on lines that I'm an expert in!)
*diaspora is still a real thing, believe it or not, and a few people use it. Trouble is, it was never a thing that worked all that well, and when the first release didn't fulfill all the wildest dreams of people who pledged to it on Kickstarter, it was already all over but the (very quiet and prolonged) shouting. In particular, failure to ship a phone app on day one seems to be a kiss of death for a new network.
Many people, myself included, are still working on the fabric of open source software that'll be needed to make the web's connective tissue strong enough to eliminate the need for the likes of Facebook and Twitter. Merely replacing the centralized networks with a decentralized one (again) is not going to be a panacea, but it'll be a step.