After reading the article posted by @khem, I explored the site a bit and found:
https://code.dblock.org/2011/07/14/github-is-your-new-resume.html
There are many factors at work, but I believe the unversal princple is at work here: the more you share, the more comes back to you. If you are genuinly trying to create and share useful contributions, it will be obvious – It is really hard to fake it on Github. The mindset that makes you successful in open source (generous, helpful, attention to detail, good communicator, etc) will also make you successful in any company.