Mastodon is a federated Twitter alternative. It is the most popular client using the ActivityPub federation protocol. It started out using OStatus, and switched to ActivityPub.
Users create an account on an instance, but can communicate with users on other instances. A full username is a user’s handle plus the name of the instance the user belongs to, for example:
If a user moves to a new instance, they can redirect or migrate their old account. Redirection sets up a redirect notice on the old profile which tells users to follow the new account. Migration forces all followers to unfollow the old account and follow the new, if the software on their instance supports this functionality. Previous posts will not be moved.
Account credentials are managed by the user’s instance, so if users forget their password, they can ask for a password reset. Whether users can delete their own accounts or not is a setting dependent on the instance admin.
An identity proof framework was added in 2019, which currently only supports Keybase. It allows users to [link their Keybase cryptographic identity](https://github.com/keybase/keybase-issues/issues/2948) to their Mastodon account.
Mastodon uses [Webfinger](https://docs.joinmastodon.org/spec/webfinger/) to translate user mentions to actor profile URIs. Webfinger specifies the path at which to find a user's profile information provided by the server.
ActivityPub federation defines a common set of APIs and formats for passing messages between servers. ActivityPub messages consist of the message itself, and a wrapper that communicates what's happening with the message, defining the "activity". The activities Mastodon implements are documented [here](https://docs.joinmastodon.org/spec/activitypub/).
Moderation takes place at the server level in Mastodon. Each instance sets its own moderation policies, either through the unilateral decisions of an admin, or through collective discussion and agreement. Admins can ban entire instances, cutting off their visibility. If an instance gets banned by many others, its users can still talk with each other, but they will be isolated from the rest of the Fediverse.
Mastodon has content warnings that admins can apply to warn users of the nature of the content before they click. Users can also report posts to moderators.
Some documented [challenges with moderation in Mastodon](https://nolanlawson.com/2018/08/31/mastodon-and-the-challenges-of-abuse-in-a-federated-system/amp/) from 2018. One recommended solution is to provide open APIs for third-party tools to help with moderation, which was [added in 2019](https://github.com/tootsuite/mastodon/pull/9387).
There is [no unified global search in Mastodon](https://github.com/tootsuite/mastodon/issues/9529), as each server monitors a different set of messages. Searching for the same keyword on different instances yields different results. The federated timeline shows public posts that the user's server knows about. Essentially, users have access to posts of people followed by people on their instance.
Posts can be public, unlisted, private, or direct. Public posts are searchable on the web. Unlisted posts are not, but can be accessed through the link. Private posts are only visible to followers. Direct posts can only be seen by the people mentioned in them. Users can also hide their network, so the lists of followers and people they're following are not visible.Mastodon's privacy settings are recommendations, not demands, so it is up to each individual server to implement and enforce them.
Federated social networks require both hosting and development costs to maintain. Each instance is funded by its own administrator and community. Mastodon’s development is funded through a Patreon run by the main developer, Eugen Gargron. It currently brings in about 70k a year, which supports him working on it full time, and covers hosting costs and a moderation team for the mastodon.social instance.
Mastodon's main initial selling point was its familiar interface that behaved like Tweetdeck. Many other federated social applications that work essentially the same way had tried and failed to get mainstream adoption prior to Mastodon, largely due to unfamiliar user interfaces.
Notable design choices in Mastodon that differ from Twitter: Instead of a 280 character limit, there is a 500 character limit. "Likes" are not broadcast to third-parties. "Retweet" and "Like" numbers are not shown until a post is clicked on. "Bookmark" is a separate feature from "Like". Sometimes unlocked accounts receive a notification to manually review certain follow requests.
User-level content controls include: Filtering on posts can automatically hide keywords and phrases. "Boosts" (like retweets) from someone can be hidden. Accounts can be muted or blocked. Entire servers, including all of its posts, can be blocked by a user. Following, muting, blocking, and domain-blocking lists can be imported.
### Interoperability
Mastodon is compatible with all federated applications that use ActivityPub. These include Pleroma, another social site, PixelFed, a photo-sharing site, and PeerTube, a video-sharing site. Mastodon users can follow a PixelFed or PeerTube user from Mastodon.
Since Mastodon is one of the most widely used decentralized social applications, there are many user requests to extend interoperability. Some of these requests may be stalled by lack of developer resources and the challenge of upgrading decentralized protocols rather than by technical limitations. Some examples of requested interoperability features:
- To comment on a PixelFed post, a user needs to follow the link to that service. A request for [remote interaction](https://github.com/tootsuite/mastodon/issues/13822) with other federated services has been requested, but is not implemented.
- A global directory for [importing friends from other networks](https://github.com/tootsuite/mastodon/issues/11886)
Mastodon.Social, the instance started by Mastodon's main developer, Eugen Gargron, initially became the server of choice for new users. The server was scaled up to a larger VPS several times, until new registrations were closed in 2018. Gargron has now started mastodon.online to support more registrations. Anyone can run a Mastodon instance, but in practice, the majority of users have concentrated within a few.
Another scalability issue is the resource requirements of Mastodon. As a monolithic web server and web app, it needs to run on a larger VPS. Mastodon hosting providers have emerged as a service to help individuals interested in being admins but without sysadmin experience to spin up servers. Pleroma, another federated social app, is a more minimal implementation that requires fewer resources to run. Rustodon is a WIP experimental Mastodon implementation in Rust, intended to address performance issues: https://github.com/rustodon/rustodon/
