I think the limitation is more technical than you make it seem. Why is there no network-level decentralization for social networks? Because P2P connections have a habit of being unstable, unreliable and hard to initiate. What if your desired peer is offline at the moment? Would be very convenient to store the information somewhere. A server in the middle just works. The primary reason not to use a server is because there is an inherent disadvantage/distrust in a central authority (à la Bitcoin). Apparently for social networks this is not an issue for 99.9% of people.