I really don't see how p2p scales better in economic terms. Because crypto currencies lack central authorities they need mechanisms that are highly energy intensive to guarantee consensus, thus driving up prices for transactions. The same thing is true for a federated social network. The sum total of all federated instances is harder to manage than facebook.
The benefit of decentralisation is redundancy and fault tolerance, not the ability to scale.
There are less energy intensive cryptocurrency scaling techniques.
true, but they'll never be as efficient as a centralised system, because intrinsic to the trustless design is a proof of work or proof of stake, which is to say the investment of resources to drive up the cost for any given actor, to make owning the system impossible. There is no way around this by design.
The platforms are going to be provided by decentralization technologies. Such as cryptocurrency, various content-centric networking, p2p everything apps, etc. Federation, where people host open services on servers and connect them is more popular now and may be a significant factor for awhile also. I assume p2p systems will eventually be more widespread once people figure out how to do it, since p2p can scale better/more easily.