https://www.techdirt.com/articles/20210303/14474346357/content-moderation-case-study-decentralized-social-media-platform-mastodon-deals-with-influx-gab-users-2019.shtml

Summary: Formed as a more decentralized alternative to Twitter that allowed users to more directly moderate the content they wanted to see, Mastodon has experienced slow, but steady, growth since its inception in 2016.

Unlike other social media networks, Mastodon is built on open-source software and each "instance" (server node) of the network is operated by users. These separate "instances" can be connected with others via Mastodon's interlinked "fediverse." Or they can remain independent, creating a completely siloed version of Mastodon that has no connection with the service's larger "fediverse."