f00f/eris/continuum/etc- The maintainers of GNU social also worked on the federated music community GNU FM, of which the most prominent instance was Libre.fm (which I also used around the same time I used GNU social). As @mattl describes it, GNU social was his effort to redirect use of GNU FM as a social network, to something better suited for social networking.
GNU social JP is a social network, courtesy of GNU social JP管理人. It runs on GNU social, version 2.0.2-dev, available under the GNU Affero General Public License.
All GNU social JP content and data are available under the Creative Commons Attribution 3.0 license.