If you would like to see what a decentralized social network looks like today, check out Leeroy. If you actually would like to interact with it, you'll need MetaMask and some Ether cryptocurrency.
Leeroy is a bare-bones fully decentralized Twitter clone. It's hard to use. It's slow. It is currently expensive, requiring one or two pennies to perform any action such as posting or following. Just last year, it was hard to fathom such a thing could exist at all.
However, this functionality represents the complete disintermediation of Twitter and the death knell of the ad-driven business model that every large technology company uses to extract profits from your private data and attention.
In the future, all social networks will be like Leeroy: a public utility that lives on the Internet, which leaves users in control of their data, fairly compensates content creators for their efforts, is agnostic and unopinioned as to the data that flows on top of it, fully community-owned and governed, and built for users.
Follow me on Leeroy at http://leeroy.io/jake.