About a year ago I became interested in why this is the case. I'm an academic computer scientist with a strong interest in the startup/web tech scene (see profile for info). My colleagues and I have been studying not just decentralized social networks, but the more general concept of decentralized architectures for personal data. This includes "personal data store" efforts which are popping up all over the place, with similarly dismal adoption, "infomediaries" which were the rage in the late 90s (before the dot com bust ate them up), etc. Overall, we've looked at around 80 companies, projects, and proposals.
If the amount of reinvention in this space is surprising, the almost wanton refusal to learn from others' past mistakes is shocking. These projects seem to do the same things wrong and fail for very similar reasons, chief among them building more technology when it's not really technology that's holding things back.
There is the widespread — but often unstated and always unexamined — belief among the participants that moving to a decentralized setting is a magic cure-all for the problems that ail today's status quo, such as privacy and interoperability. Sadly, this belief is simply wrong.
I certainly sympathize with the urge to pat these guys on the back for trying, but is it really courage or foolhardiness? If 10,000 amateurs have tried to solve P =? NP and failed, do we encourage the 10,001th guy to give it a shot as well, or do we tell him to learn some math and CS first, and gain an appreciation for why the problem is hard and probably not worth taking on unless you really really know what you're doing?
[Our study is not out yet, but feel free to contact me if you're interested in discussing this.]