This service uses a Free microblogging tool called StatusNet. StatusNet is available under the GNU Affero General Public License Version 3.0, a Free Software license for network services.
You can get a copy of the software from the StatusNet main site. The version of the software that runs on this site is heavily modified from that version. You can find the modified software on GitHub.
The site also depends on certain libraries and other software; you can get those at the StatusNet site, too.