I believe it will be faster to search a potential employee by targeting the job add to him.
If someone looking for a job passively and he opens a particular question to see the answer in stackoverflow, then chances are more that ad being viewed if it is related to him.
i.e. If I am looking at question related to WCF (i.e. TAG with WCF,.NET ...) then there are more chances that I click on the job if it is related to the TAG of questions WCF not RUBY! (this is just an example)
I think so it is fairly straightforward to achieve this and will be more fruitful when there will be more jobs on the stackoverflow job-board.
Same thing can be looked for other ads around stackoverflow.