ActivityPub

Email works on a simple idea: it doesn’t matter which provider you use. An address at one provider can write to any other address, because everyone follows the same rules. ActivityPub applies exactly this idea to social networks. It is a defined set of rules for how programs pass posts, likes and subscriptions to one another. As a result, someone on a microblogging service can follow someone on a video platform, as long as both support these rules. In technical terms, such a set of rules is called a protocol.

Why there is no single operator in charge

At the big social networks, everything belongs to a single company. It decides which posts you see, which ads are shown, and who gets banned. If you don’t like it, all you can do is leave — and lose all your contacts in the process. With ActivityPub, this constraint disappears entirely.

That’s because here, it isn’t one company running a single giant service; instead, many people and organizations run thousands of small servers. These servers are called instances. Each instance has its own rules, its own moderators, and often only a few hundred members. Even so, they are all connected to one another because they speak the same language. This overall network of connected instances is called the Fediverse.

The practical advantage: you can switch servers and take your followers with you. This is reminiscent of being able to keep your phone number when you switch providers. For many people, this is the main reason they find the principle interesting at all. It’s an attempt to make social networks independent of individual corporations.

What gets sent back and forth between servers

Your address on the network consists of two parts: your username and the server you’re on. It looks roughly like an email address. If someone on another server wants to follow you, their server sends a message to yours. Your server keeps a record of this subscriber in a list.

If you then write a post, the following happens: your server goes through the list and sends a copy to every server where one of your followers is located. Each post is essentially a small data packet with defined fields — who did what, and when. A like or a share is technically the same thing, just with a different entry in the field for the action. That’s why a video service can also understand comments coming from a microblogging service.

A common misconception: posts are not stored in one central location. Rather, they are duplicated and distributed across many servers. This has a drawback worth knowing about. If you delete a post, your server has to send a deletion signal to all the others. Whether they actually comply with it is not something you can enforce.

Mastodon, Threads and other familiar names

The best-known service using ActivityPub is Mastodon, a microblogging service similar to X, formerly Twitter. There is also PeerTube for videos, Pixelfed for photos, and Lemmy for discussion forums. All these services look different, but they exchange the same data packets. Because of this, you can subscribe to a PeerTube channel from Mastodon.

The term has been showing up in business news ever since Meta partially connected its Threads service to ActivityPub. Overnight, a niche topic for tech enthusiasts turned into something with millions of potential users. The European Commission and various government agencies also run their own Mastodon servers. They no longer want to distribute their announcements solely through platforms owned by private corporations.

However, ActivityPub should not be confused with other open networks. Bluesky, for example, uses its own protocol called the AT Protocol and is therefore not technically a direct part of the Fediverse. Connections between the two worlds only exist through additional translation services, so-called bridges.