I'm using Linux Mint at the moment. I chose it after upgrading Ubuntu to 12.04 when things became unstable. Linux Mint is pretty good so far. It's stable, and uses a mix of Gnome 3 and MATE for the desktop environment.
The obvious answer to why is, no one's built it yet. Forrst supposed fill the showing off things in pictures void (plus it's exclusive, just like Facebook used to be). Github comes pretty close, you can even have a personal page (though updating has be done via pushes).
A feature I'd like to see in such a site is a repository/tool for defining syntax rules & colorschemes for editors.