Currently, it seems to be ActivityPub, which projects like Mastodon use.
Coming back to this statement:
> With an open standard, I don't see why you can't host your own social network
You also need:
- photo hosting
- video hosting
- comments
- timelines
- feeds
- search
- social graphs
- (etc.)
for a social network to have any relevance.
I agree with this point: usability is often not affected by what happens in the back. However, with open standards, it would be possible that people create other social networks because of protocols.
You mention all these technical problems that have to be solved to create a social network. I can see someone building an open source framework for all of these. For example with personal websites, everything except social graphs and feeds already exist.
Mastodon, Scuttlebutt to name a few. They are somewhat popular, but they are bad at many of the things I described.