Notice cordite said a Github mirror, not the Github mirror. You don't need to have your main development happen on Github to have a mirror there, just someone mirroring from the main server.
to be fair, i assume he's only asking because github starring something is actual a fairly good way of keeping track of projects in the way he's talking about. i agree that it sounds a bit presumptuous to say "where's the github mirror, mr. unpaid contributor to open source project", but on the flip side it's a fair point in terms of potentially capitalizing on exposure for a project. and also it's very true that you can sometimes find the best code in very unassuming places.
I'd be really interested to see statistics on this. I don't work in Silicon Valley, and almost all of the stuff people around me are publishing is on GitHub.
Where's a github mirror?
A google search reveals some entries from the language implementations.
Go: https://github.com/jbarham/go-cdb Java: https://github.com/malyn/sg-cdb Haskell: https://github.com/adamsmasher/hs-cdb