ActivityPub

ActivityPub is a decentralized social networking protocol standardized by the World Wide Web Consortium (W3C) in 2018. It provides a client-to-server and server-to-server API for creating, updating, and deleting content across federated platforms, enabling interoperability between independently operated services. ActivityPub is the protocol that powers the Fediverse, and its design reflects a specific theory of platform governance: rather than regulating platforms, replace them with protocols.

The protocol's data model is built around activities — verbs like Create, Follow, Like, Announce — that actors perform on objects. This activity-stream model is flexible enough to support diverse platform types (microblogs, video hosts, social networks) while remaining standardized enough to ensure cross-platform communication. An ActivityPub implementation can be as minimal as a single-user server or as complex as a large-scale instance with thousands of users.

ActivityPub's limitations are as instructive as its successes. The protocol does not specify content moderation mechanisms, leaving each instance to develop its own policies. It does not address data portability at the level of social graphs. And its server-to-server federation model creates single points of failure: if a popular instance shuts down, its users' social connections fragment. The protocol governs interoperability, not sustainability.