Mastodon is a federated social network, with similar features to Twitter, but administrated as a decentralized federation of servers running open source software. Users belong to a single Mastodon server, known as an "instance", where they post short messages for others to read, subject to the privacy settings of the user and instance. The service seeks to distinguish itself from Twitter through its orientation towards small communities and community-based, rather than top-down, moderation. Like Twitter, Mastodon supports direct, private messages between users, but unlike "tweets" posted on Twitter, Mastodon "toots" can be private to the user, private to the user's followers, public on a specific instance, or public across a network of instances. The network of federated Mastodon instances forms one part of the Fediverse, a federated network that includes servers running any social network software that uses OStatus.
The Mastodon mascot is a sitting Proboscidean using a tablet or smartphone; however, the distribution of fur is more suggestive of a woolly mammoth than the mastodon.
Mastodon approximates the user experience of Twitter and uses an interface similar to TweetDeck, a professional Twitter application. On both services, users post short status messages for others to read. These messages can include up to 500 text characters, an extension of Twitter's 140-character limit, and are known as "toots", "noots", "awoos" or other terms instead of "tweets", as on Twitter.
Users join a specific Mastodon server, known as an "instance", rather than a single flagship website or application. The instances are connected as nodes in a network, and each server can administrate its own rules, account privileges, and whether to share messages to and from other instances. The flagship instance, Mastodon.social, had about 42,000 users as of early April 2017. Other instances are based on communal interests, such as Internet memes, Minecraft, or technology. The global use is rising fast, having about 290,000 users as of April 16, 2017.
The service includes a number of privacy features. Each message has a privacy option, and users can choose whether the post is public or private. Public messages display on a global feed, known as a timeline, and private messages are only shared on the timelines of the user's followers. Messages can also be marked as unlisted from timelines or direct between users. Users can also mark their accounts as completely private. In the timeline, messages can display with an optional "content warning" feature, which requires readers to click on the content to reveal the rest of the message. Mastodon instances have used this feature to hide spoilers, trigger warnings, and not safe for work (NSFW) content, though some accounts use the feature to hide links and thoughts others might not want to read.
Mastodon aggregates messages in local and federated timelines. The local timeline shows messages from users on a singular instance, while the federated timeline shows messages across all participating Mastodon instances. Users can communicate across connected Mastodon instances with usernames similar in format to full email addresses.
In early 2017, journalists distinguished Mastodon from Twitter for its approach in combatting harassment, one of Twitter's largest issues. Mastodon uses community-based moderation, in which each instance can limit specific content. For example, the flagship instance Mastodon.social bans content that is illegal in Germany or France, including Nazi symbolism and Holocaust denial. Instances can also choose to limit messages with discriminating content. The service's founder believes that small, close communities would police toxic behavior more effectively than a large company's small safety team. Users can also block and report others to administrators, as on Twitter.
While Mastodon was first released in October 2016, the service began to expand in late March and early April 2017.The Verge wrote that the community at this time was small and that it had yet to attract the personalities that keep users at Twitter.