Pleroma is an open source, federated social network implementation compatible with Mastodon and Gnusocial. It focuses on being a light and generalized implementation of ActivityPub, as opposed to Mastodon, which focuses solely on microblogging. Pleroma has support for a variety of ActivityStreams Objects, including Note, Article, Page, Image, and Video.
The codebase has a clean separation between server and client, so an admin can install multiple frontends. In addition to the default PleromaFE, pleroma comes installed with a customized mastodon frontend, called the MastoFE. The Pleroma contributors have said their goal is for Pleroma to be a unopinionated social server that can fill multiple niches, e.g. Twitter-like, Facebook-like, Instagram-like, etc.
Instances can be found on various tracker: