Mastodon
Also known as: ActivityPub, fediverse, Mastodon instance, federated social media, decentralized social network
Mastodon is an open-source, decentralized social networking platform where the software is run by thousands of independent servers (called instances), each with its own rules and moderation, yet able to communicate with each other.
It is part of the 'fediverse' (federated universe): a collection of platforms (Mastodon, Pixelfed, PeerTube, etc.) that share the ActivityPub protocol, allowing users on different platforms and instances to follow and interact with each other. Mastodon saw significant growth following Twitter's turbulent post-Musk period, gaining millions of users in late 2022. It remains niche relative to mainstream social networks but represents the most significant real-world deployment of decentralized social media architecture.
Mastodon, explained at three levels
At home
Mastodon is a social network similar to Twitter, but with one important difference: instead of one company controlling the whole platform, Mastodon is made up of thousands of independent websites (called instances or servers), each run by different people for different communities. When you join Mastodon, you pick an instance to join (like mastodon.social for general users, or fosstodon.org for open-source enthusiasts). You get a username on that instance (like @name@mastodon.social), but you can still follow and interact with people on other Mastodon instances.
At work
Mastodon is relevant professionally for technical and open-source communities, academics, journalists, and people concerned about big-tech platform control over public discourse. Many developers, researchers, and digital rights advocates have established a presence on Mastodon. For most businesses, Mastodon's relatively small user base and the technical friction of onboarding make it a secondary consideration behind mainstream platforms, but for companies serving developer, academic, or open-source audiences, maintaining a Mastodon presence builds credibility with those communities.
At home
Mastodon's federated model has practical implications: your posts appear on your home instance and can be seen by followers across other instances, but each instance has its own moderation rules and can 'defederate' from instances that violate their standards (blocking all interaction with a problematic instance). This means Mastodon does not have centralized content moderation: each instance moderates its own community, and the instance you choose affects what content you see and what instances you can interact with. Choosing a well-moderated instance with reasonable rules is important for the Mastodon experience.
At work
Organizations running their own Mastodon instance can host a verified, branded presence that they fully control: no terms of service changes from a central company, no algorithmic feed changes, and their posts appear under their own domain. This is appealing to organizations concerned about platform dependency: a government agency, university, or media organization running mastodon.myorganization.org retains full control. The technical overhead of running a Mastodon instance (server management, software updates, moderation) is significant, but managed hosting providers (such as masto.host) reduce this burden.
At home
The ActivityPub protocol that powers Mastodon and the fediverse represents a potential model for a more decentralized internet: applications built on ActivityPub can communicate with each other regardless of the company or server hosting them: similar to how email works (you can send email from Gmail to Outlook to Yahoo without needing one company to control all email). Meta's decision to build Threads with ActivityPub compatibility suggests the protocol may achieve mainstream relevance. The fediverse vision is that no single company can lock users in or shut down the entire network.
At work
From a content moderation research perspective, Mastodon's federated model is a natural experiment: can distributed, community-led moderation (each instance moderating itself, with defederation as the enforcement mechanism) effectively manage harmful content better or worse than centralized algorithmic moderation? Research findings are mixed: federated moderation can be more responsive to specific community norms, but the defederation mechanism creates fragmentation, and instances with inadequate moderation resources can become hosts for harassment or extremism before defederation occurs. The model trades central control for community autonomy.
Real-world examples
At home
Sophie was a software developer unhappy with Twitter's post-2022 changes. She created a Mastodon account on fosstodon.org (the open-source focused instance) and found an active community of developers who matched her professional interests better than her Twitter feed had in years. She follows colleagues across multiple Mastodon instances and even some Threads accounts through the ActivityPub federation. She misses the journalists and policy people from Twitter but found Mastodon's developer community more valuable for her professional needs.
At work
Thomas worked for a public research university that was evaluating its social media presence post-Twitter. Rather than following users to any single commercial platform, the university set up its own Mastodon instance (social.university.edu) for its academic community. Faculty could create verified accounts under the university's domain, posts appeared across the fediverse to their followers, and the university retained control over its community without dependence on any commercial platform's terms of service or moderation policies.
Common confusions
- Many people assume Mastodon is just one website, like Twitter is one website — Mastodon is software, not a single site: thousands of independently operated websites run Mastodon software and form a network. mastodon.social (the largest general instance, run by Mastodon's creator) is the most prominent but not the only one. Your username on Mastodon includes your instance (like @sophie@fosstodon.org vs. @sophie@mastodon.social), and these are two different accounts on two different servers that can nonetheless follow each other. Choosing which Mastodon instance to join is part of the onboarding experience and a source of friction for new users.
- Some assume Mastodon's decentralization makes it completely censorship-resistant — Mastodon instances are run by people who can and do apply content moderation rules: typically more focused on their specific community norms than Twitter's broader policies. If the instance you join bans your content, you lose your account on that instance (though you can move your account to another instance and take your followers). Additionally, instances can defederate from each other, cutting off cross-instance communication. Mastodon is resistant to single-company censorship but not to all moderation: distributed moderation by community administrators is its model.
For analysts
For most business clients, Mastodon is not an immediate priority: the user base is significantly smaller than mainstream platforms and the audience skews toward tech-savvy users. The exceptions: organizations serving developer, academic, or digital rights communities where Mastodon presence builds credibility; organizations specifically concerned about platform dependency on commercial social networks; and technology companies monitoring the ActivityPub ecosystem for interoperability opportunities as Threads federation matures. For these clients, maintaining a presence (or running their own instance) is worth the modest investment.
History & origin
Eugen Rochko released Mastodon in 2016 as an alternative to Twitter with decentralized hosting. The platform grew slowly in tech and open-source communities until two major Twitter migrations: the first when Twitter banned various accounts in 2018-2019, and the much larger migration following Elon Musk's Twitter acquisition in late 2022, when Mastodon gained more than a million new users within weeks. The post-Twitter migration stabilized below initial expectations as many users discovered the friction of choosing instances, setting up accounts, and finding existing communities. Mastodon and the broader fediverse gained mainstream attention when Meta announced Threads would integrate with ActivityPub: a major platform choosing to federate with independent servers rather than build a closed ecosystem.