Bluesky, the social media site that launched into an invite-only beta about 2 years ago, is now accessible to the public without the requirement of an invitation code.
The platform was initially a project of Twitter before being spun out to be its own thing.
Bluesky are not part of the Fediverse that Mastodon, PeerTube, Pixelfed, etc are using, ActivityPub. They have their own protocol "AT Protocol" but like ActivityPub, servers would be able to follow and interact with each other.
The idea at the time was for Twitter to build the AT Protocol as part of Bluesky and then have Twitter use that technology. Thankfully, that was all put to a halt once Elon Musk purchased Twitter in October 2022.
At the time of writing, Bluesky is the one and only instance of the AT Protocol, so I would hedge a bet that now that Bluesky is public, they would be keen to release AT Protocol to the world for all to use.
I can be found on Bluesky @pf.ie, but you won't find me posting that much on it.