What Is ActivityPub?
ActivityPub is an open protocol — a W3C standard published in 2018 — that enables decentralized, federated social networking. It is the backbone of the Fediverse: the interconnected constellation of platforms that includes Mastodon, Pixelfed, PeerTube, Lemmy, and dozens of others.
Think of it like email. You can have an address on Gmail and send a message to someone on Outlook. Neither service needs to be the same company. ActivityPub does the same thing for social media: it lets users on completely different platforms follow each other, like posts, and reply across servers.
The Key Concepts
Actors
In ActivityPub, every user account is an Actor. An actor has a unique URL as its identity. When you create an account on a Mastodon server, your profile becomes an actor with a publicly accessible JSON-LD document that other servers can query.
Activities and Objects
Everything that happens is an Activity — Create, Like, Follow, Announce (boost), Delete, and so on. Activities act on Objects like Notes (posts), Articles, or Videos. This vocabulary is defined in the Activity Streams 2.0 specification.
Inboxes and Outboxes
Each actor has an inbox (receives activities from others) and an outbox (publishes the actor's own activities). When you follow someone on a remote server, your server sends a Follow activity to their inbox. When they accept, their server starts forwarding their posts to your server's inbox.
How Federation Works Step by Step
- You sign up on
mastodon.socialand follow a user onfosstodon.org. - Your server sends a Follow activity to the remote user's inbox URL.
- The remote server accepts and records the relationship.
- When that user posts, their server sends the new Note activity to all followers' inboxes — including yours on
mastodon.social. - Your server displays the post in your home timeline.
Why This Architecture Matters
| Centralized Platform | ActivityPub Federation |
|---|---|
| One company controls all data | Data is distributed across many servers |
| Banned from one = banned from all | Move servers without losing your network |
| Platform can shut down entirely | Other instances survive independently |
| Proprietary API, can change anytime | Open standard, interoperable by design |
Platforms Using ActivityPub Today
- Mastodon — microblogging (Twitter alternative)
- Pixelfed — photo sharing (Instagram alternative)
- PeerTube — video hosting (YouTube alternative)
- Lemmy / Kbin — link aggregation (Reddit alternative)
- Writefreely — long-form blogging
- Funkwhale — music and podcast hosting
ActivityPub is not just a technical curiosity. It is a working, deployed standard that millions of people use daily — proof that a decentralized social web is not just theoretically possible, but already here.