GitHub achieving it’s goal of social coding based on an awesome version control system is one (amazing) thing, but the byproduct of your profile acting as a resume is simply stellar.
I’ve been in hiring positions before, and resumes are by far the worst thing I’ve had to deal with when trying to figure out if an applicant should be deemed considerable for the position at hand. GitHub gives more insight than anything in my opinion. It’s far too easy to fly under the radar in your resume. Past job titles can seem impressive, but you only have to be in this industry a short time before realizing that they really don’t mean a thing in most cases. The proof is in the pudding and GitHub is a fantastic place to focus your effort, even if you’re not actively job searching.