ActivityPub is a decentralized social networking protocol based upon the ActivityStreams 2.0 data format. It provides a client to server API for creating, updating and deleting content, as well as a federated server to server API for delivering notifications and content.
ActivityPub is already implemented and deployed to a number of projects and a wide userbase, including over 1 million registered users across thousands of Mastodon instances. ActivityPub allows software projects both small and large to build social network offerings into their systems. Adding ActivityPub support allows interoperable social networking between applications with entirely different codebases. For example, Mastodon and PeerTube users are able to use ActivityPub to allow users to share videos and comment across different servers.