Mastodon

Mastodon is a free and open-source decentralized microblogging platform. It was launched in 2016 by Eugen Rochko as an alternative to centralized social networks. Rather than a single site, Mastodon operates as a federation of independently run servers called instances. Each instance hosts user accounts and sets its own rules and moderation policies.

The platform uses the ActivityPub protocol to enable interoperability with other ActivityPub-enabled services. Users on one instance can follow and interact with users on other instances, and posts propagate across the federated network to followers on different servers. Moderation is primarily at the level of individual instances, allowing communities to enforce their own standards.

Posts, called toots, can include text, images, videos, or polls, within a typical 500-character limit. Users can attach media, use hashtags and mentions, and apply content warnings or spoiler text. Visibility settings include Public, Unlisted, Private, and Direct. The interface provides Home timelines, Local timelines for the user's own instance, and Federated timelines that show activity from outside.

Mastodon software is released under the GNU Affero General Public License version 3 (AGPL-3.0). The decentralized nature means there is no central owner; communities run their own instances with varying policies. Because it is part of the Fediverse, Mastodon interoperates with other compatible services.