Joey Hess' github-backup backs up everything github knows about a repository (including branches, tags, other forks, issues, comments, wikis, milestones, pull requests, watchers, and stars) to the repository.
I'm sure most of these could be re-imported into an alternative service through its API, even if with some loss of fidelity.
(for others: he's also the creator of git-annex, an out-of-band file storage extension to git, and for a long time was the maintainer of critical parts of Debian's ecosystem. He likes to live in a yurt: http://joeyh.name/yurt/)
I'm sure most of these could be re-imported into an alternative service through its API, even if with some loss of fidelity.
https://github.com/joeyh/github-backup