The ActivityPub protocol 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.
Probably the best known implementor of ActivityPub is Mastodon (you can find me @murb@todon.nl), but as Eugen Rochko writes on the Mastodon blog, there are many.