The biggest social outlets these days are megaladon-sized companies with millions of users interacting in small clusters. Facebook is a great example of those clustered social interactions; you’re only friends with your friends and can’t see anything outside of that cohort (for the most part).
Twitter is a little different in that you can deliberately create a cohort of people you want to interact with and consume information from, but you can see the wider ecosystem’s data since most people on the platform have public profiles.
There are plenty of other examples of this that ape decentralisation. Slack apes a decentralised model by allowing your email address sign into multiple Slack instances for different teams. I’m a member of a Slack group for my work, for some freelance stuff I help with and some tech communities, for example. In the gaming world, Discord does a similar thing.
But none of these are actually decentralised. You have one login for multiple places to engage. The best example of truly decentralised social interactions is IRC, which has waned in terms of usage but the basics of how IRC work permeate throughout most of our modern social platforms.
IRC is an open & totally decentralised construct. There’s no single “IRC” to join. You use the IRC set of protocols to join various servers, mostly through a single client like mIRC or LimeChat. The reason Slack has overthrown IRC as the defacto workplace chat, despite having basically the same functionality, is ease-of-use and a nicer app.
IRC used to be the go-to social construct and is still hugely popular, particularly with more technically-savvy types. It’s not as ubiquitous as Slack in workplaces, and not as ubiquitous as Facebook for mates. But it does define the basic rules for a decentralised social network.
Newer technologies like Mastodon are trying to bridge the gap between one login with a clean UX and a decentralised network effect. I’d love to see them take-off, but the majority of users are still likely to stick around the current systems that are so deeply ubiquitous.