(Although there is a problem if you want to make nice things and get paid for them...)
There already are P2P social network platforms, the problem is how do you get mass adoption?
The actual cost of what e.g. FB does for its users is tiny.
What the magnitude of such metric actually is, probably depends as much on cultural aspects of your target demographic as the quality and usefulness of the network provided services.
Surely there is an academic paper on the subject, but if there isn't, then the real question is Why?!
There are various ways to get it. I would say viral expansion is the best way.
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Imagine a de-centralized Facebook with the same engagement-optimized feed. Maybe it runs over the Web, maybe it runs on something like Ethereum.
What if we built that, and wound up with the same set of problems? Except now there's no off button, nobody to petition, and government can't intervene.
Decentralization is not a silver bullet. And in this case I might worry that the medicine is worse than the disease.
Well, there is someone who is responsible, but the problem is that you cannot hold them accountable. Which is why democracy avoids the same concentration of power in the first place.
Which is also the whole point of decentralization. You avoid the concentration of power that would allow any single party or small minority to turn against you without any way out for you.
If you email provider starts selling your emails to the highest bidder, you don't need to regulate your email provider. You simply switch to a different one. Or run your own server. Decentralization is about keeping the power your hands.
I'm a big fan of decentralization in general, but I think it's worth pointing out where things could go wrong at scale.
Look, you are writing this on YOUR domain because Wordpress gave you that ability. It powers 20% of all new websites. What we need as a society is a Wordpress for social networking.