Why would it mean the project is "dead"?
Do you mean that community projects are by nature "dead" projects?
What does "dead" mean, exactly?
My OS is a community project. OSX/iOS are built from community projects. Lots of scripting languages are community projects. Mozilla is a community project. Wikipedia is a community projects. I could go on.
I realise I may think a bit differently than many programmers, but I care less about how much a particular chunk of code is actively changing than whether it works really well over the long term (simple, stable, reliable, secure). I like "timeless" software than quietly continues to work for many years, remaining relatively unchanged. In my experience, well-engineered software like that is often immune from so-called "bit rot". Because it was designed correctly, with minimised complexity and maximised portability as a top priorities, from the beginning.
From a design and implementation perspective, there are no real impediments to a decentralised social network that cannot be overcome. However first you have to decide what you mean by "social network"? Does it have to be a clone of FB or G+, save for the centralisation element? Or does your definition allow some changes to their approach? For example, what if the network was private? What if there were no ads? What if it was comprised of lots of smaller networks of maybe 100-200 people (like your "friends" on FB) instead of being one massive, public image gallery/chatbox? What if it didn't require the web, as FB does? What if it was application-agnostic?
What do you demand from a "social network"?
Does it have to be a FB/G+ clone?
Anything is possible, so to speak. But not everything is necessarily ready to be received based solely on technical merit. How much marketing and PR is needed?