fosstodon.org is one of the many independent Mastodon servers you can use to participate in the fediverse.
Fosstodon is an invite only Mastodon instance that is open to those who are interested in technology; particularly free & open source software.
If you wish to join, contact us for an invite.
And while we are at it, there are no official Facebook groups either and there never will be. If you are looking for the official OctoPrint community you can find that on community.octoprint.org (forum) and on discord.octoprint.org (Discord server).