From https://git.gnu.io/gnu/gnu-social/tree/master:

GNU social is a free social networking platform. It helps people in a community, company or group to exchange short status updates, do polls, announce events, or other social activities (and you can add more!). Users can choose which people to "follow" and receive only their friends' or colleagues' status messages. It provides a similar service to sites like Twitter, Google+ or Facebook, but is much more awesome.

The continuation of StatusNet, which was the codebase behind identi.ca until the owner wrote pump.io in nodejs and migrated to that.

A federated social web implementation