ActivityPub used by Mastodon
ActivityPub is an open, decentralized social networking protocol based on Pump.io's ActivityPump protocol. It provides a client/server API for creating, updating, and deleting content, as well as a federated server-to-server API for delivering notifications and content.Project status
ActivityPub is a standard for the Internet in the Social Web Networking Group of the World Wide Web Consortium (W3C). The standard was co-authored by Evan Prodromou, creator of StatusNet (now known as GNU social).  At an earlier stage, the name of the protocol was "ActivityPump", but it was felt that ActivityPub better indicated the cross-publishing purpose of the protocol. It is the most widely supported standard (by some margin) in the Fediverse.In January 2018, the World Wide Web Consortium (W3C) published the ActivityPub standard as a Recommendation. Lead author Christine Lemmer-Webber notes that the team predominantly identified as queer, which led to features that help users and administrators protect against "undesired interaction." She also notes that the team authoring ActivityPub had no corporate participation.The W3C Social Community Group organizes a yearly free conference called ActivityPub Conf about the future of ActivityPub.Former Diaspora community manager Sean Tilley wrote an article that suggests ActivityPub protocols may eventually provide a way to federate Internet platforms.