(OK, this is an old article, but I just noticed it.) Mastodon is not a social network, but a “federation” of social networks. It is built on open-source protocols and software, which you can use to set up your own social-network node. You can then become part of the “fediverse” by associating with other Mastodon nodes, allowing users of one to interact with users of another. What is allowed on a node is entirely up the policies of the node admins. The Fediverse is intentionally decentralized, and admins of one node do not have the power to control what users do on another node. Contrast this with traditional social-networking sites like Facebook and Twitter, each of which is controlled by a single company. Naturally, such autonomy is a double-edged sword. So what do you do when the neo-Nazis move in? Gab, home of those who have promoted violence against various groups based on ethnicity, sexuality etc, is now a Mastodon node. How is the rest of the Fediverse dealing with them? The answer seems to be: you can’t stop them operating, but you can shun them. <https://www.theverge.com/2019/7/12/20691957/mastodon-decentralized-social-network-gab-migration-fediverse-app-blocking>