For me, I push my toy projects there - my first RoR app, and that kind of stuff. Would I like an employer to decide hire/no hire based on my weekend projects? Thanks, no.
GitHub profile is a nice add on for a job application, but it shouldn't be the whole job application.
By far the most interesting work I'm doing is for my current employer; and it's not OSS (and wouldn't make sense to open source).
In theory, I could cut back on my work-work, and spend more time on side projects; but I have a family, and other interests as well. I'd rather have good overlap between "interesting work" and "work I'm paid for" than sacrifice my time with my kids or my guitar so I can work on interesting technical problems I'm not paid to work on.
For some people this could work out well -- e.g., maybe the crucial and awesome component of your company's infrastructure that you're building can be open-sourced! -- but for lots of other people life simply won't line up that way, and building something serious (but a personal project) on GitHub in your spare time means you don't have spare time for anything else in life... I wouldn't demand that of myself, or of anyone I was hiring.
We will add another form of social sign up soon so that people who do not use GitHub can sign up - most likely Twitter.
Honestly, for anything "career," LinkedIn should be the norm unless their API has unsurmountable technical problems.
Quite frankly, I don't like tying my career to my github account. My best work is closed-source!