Recruiting hackers on GitHub vs StackOverflow

My start-up is looking to hire software engineers in Downtown Palo Alto. We're going through a batch of resumes now and looking to send out another job post.... Has anyone recruited on GitHub or StackOverflow? If so, what were your experiences doing so? Which one would you recommend? (if either) I thought posting jobs on a social software dev site would be more efficient than posting on a site like linkedin...Any thoughts? john@webaction.com