Freenet[1] does actually have a decentralized social network plugin, sone[2]. It's not exactly user friendly, but at least it seems to prove that if people care about these things enough, they will use a solution even if it is difficult. By making the user experience simpler (less technical), one should be able to lower the bar significantly. It's worth noting that everything in the sone-guide[2] could be done automatically; freenet just isn't designed for simplicty.
[1]https://freenetproject.org
[2]http://freesocial.draketo.de/sone_en.html