This service is still in Alpha release but is already deployable and usable, and federates with other Fediverse servers.
However, there is no “main” instance you go to join. The intention really is that you host your own instance for yourself and a few friends and family. To this end, it is designed to be very lightweight and will happily run on a Raspberry Pi or even a $5/pm VPS.
This is taking a very different approach from say Mastodon which has one main instance everyone could join, but then it sits with the issue that everyone joins there, and it becomes a bit “centralised”. GoToSocial has been designed as lightweight for self-hosting, and also has a Docker image installation, so it makes it really easy for (and encourages) most people to host their own instance.
It seems to also be focussed very much around privacy (defaults to unlisted posts) and permission controls (for example, you have an option to post to mutual-only where both people follow each other). Also, by hosting your own service you set the rules, and you are also your own admin. You can choose to turn off likes, replies, boosts, etc as well. Being your own admin also means you can easily adjust the post length as well.
It does conform to the Mastodon API so apparently some Mastodon clients will also work fine with it.