Projects can also be mirrored on Github[1], so even if the main repo is somewhere else (e.g. GitLab or Bitbucket) contributors can still be credited via Github[2] and the project can still get the publicity on Github.
[1]: https://help.github.com/articles/about-github-mirrors/
[2]: e.g. https://github.com/dsandler?tab=contributions&from=2015-10-2...