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.
Pleroma is a free, federated social networking server built on open protocols. It is compatible with GNU Social, Mastodon, and many other ActivityPub and OStatus implementations.
The project consists of several components: Pleroma is the server implementation, and comes bundled with PleromaFE, the default frontend. Other useful utilities are also provided, such as an ActivityPub relay.
This is pump.io. It's a stream server that does most of what people really want from a social network.