Created by Eugen Rochko (@Gargron) because he was annoyed with Twitter and it's series of change, Mastodon is a "Twitter like" social network (with a TweetDeck inspired design) that anyone can host on his dedicated server but instead having separated websites with no link or only one big website, every Mastodon Server (called instances) can be linked with another simply by following another person from another instance.
The initial launch was 6 months ago but this week, an important french community joined this project, creating their own instances and their accounts. This created an overload on the official Mastodon server, Mastodon.social, that forced his admin and creator to close (temporarily?) registrations. In 48h, more than 30 000 users registered on the main server, causing huge slowdowns and crashes. Hopefully, because of the decentralized structure, anyone have been able to register on another instance and still see every Toots (Tweet equivalent) from Mastodon.social.
To easily find another Mastodon instance, @TheKinrar, the hoster of the 2nd most used instance Mastodon.xyz, created a tool, Instances.Mastodon.xyz, that shows a list of instances with useful infos like how many users are registered, if it's opened to registrations, uptime, HTTPS security level and IPv6 support.
This have been covered by a lot of french medias like Le Monde, 20Minutes.fr, Numerama, Korben.info and also by The Verge.
Unfortunately, Mastodon is not quite user-friendly because of it's unusual "way to do things". That's why another user @Fawn created a helping guide called MastoGuide.info that explains a lot of things specific to Mastodon like instances, how to mention a user that is not on your instance...
Mastodon have a lot of functionalities and it would take too much time (and lines) to explain so you can read the official user guide on GitHub and MastoGuide.info.
Sources: GitHub Repo The Verge Le Monde 20Minutes.fr Numerama Korben.info
Side note: Because I'm French, my English is not perfect. So if you see some typos, let me know !