In all honesty, we don't look to see if a possible candidate is active on either of those sites. That's not saying they aren't important. Just as a Designer can show a portfolio of his work, a developer can use Github as a portfolio of her development. The thing is, unless they are doing this work on the side, chances are the code from their previous/current employer won't be in a public repository. Stackoverflow is good to see if the person gives good advice and knows how to troubleshoot but the questions we ask in interviews do the same thing. We also have a quick test for interviewees to take just to see just how good they might be.