Remember social games like Second Life? Vircadia, was created from a failed spiritual successor to it and it's free and open source too. Originally known as Project Athena, they've recently begun making some public announcements to bring more attention to it.
The idea is to have a big shared 3D social space that supports both normal and Virtual Reality PC hardware with support for Linux, macOS, Windows and Android. Unlike other social stuff, since it's open source it can't just be shut down and anyone can use it and it allows people to create their own avatar, their own worlds and much more.
As for the current status, they say it's "around an early beta". Functional, usable and stable enough with the core functionality there but it still needs a huge amount of polish and plenty of work ahead. They also seem keen to note how Linux is a properly supported platform, with some developers using it as their primary system.
What do you actually do? What's possible? A lot it seems. Socialise with friends, make games, watch movies, even use it for work meetings and more. It's not limited to any one specific thing and you can build / script it live in-game with others so you can shape it to your liking.
See more about it on the official site.
We'll be following this along a bit closer now to see what happens with it over time, could be great.