Ads can catch good people in transition, or looking for a change.

Anecdotally, I've hired truly great people, and interviewed ex-Googlers and other good pedigrees, from Stackoverflow Careers and HN posts.

1) I once hired a pre-Googler on StackOverflow even - a guy who was on his way to Google in the fall, but liked my project, and wanted a summer gig. He intended to spend a few months vacationing, but got bored.

2) Another guy I hired via HN originally just left for Facebook, this month :(

3) And our current, long term, best developer, I found on Careers StackOverflow. I credit him with much of the success of our company. He's a special combination of outdoorsy and amazing coder, but no college or professional pedigree. He was hurling dynamite at avalanche banks when I found him, coding amazing apps with little revenue potential.