Built With: Other
The new kid on the block but not that young since it was started sometime in 2017. In some quarters it's been dubbed the "Twitter without Nazis". But that is just a fancy headline meant to gain clicks.
Mastodon is both a microblogging site and a microblogging script. It has instances or individual installations where anyone can join and start posting. Not happy with any of the communities? You can download the gem and install it on your server and start using. That's the ethos of the Open Microblogging Standard that the site follows (an inspiration from GNU Social which in turn was once called StatusNet).
The site really looks cool. Steely dark with some sort of gradients. The layout is boxed with the "content box" centered making readability to be easy.
The text color varies from white to a an easy-on-the-eye shade of grey.
When logged in the site looks a lot like Tweetdeck's dark version.