It is an interesting conundrum. I see the same job, for the same salary, posted again and again. I can see, if you're a poster, that you might assume that someone, who might be perfect for the role, might not have seen the job that time around, so it might be worth advertising the same job again.
But I'm seeing jobs that are just being re-advertised for months on end, at the same salary. If someone were looking for that job, don't you think they'd have seen the ad by now?
It leads me to an interesting question - at what point do you, as a recruiter, throw your hands up in surrender and accept that there whilst there might be nobody sufficiently skilled to do the job, there might also be something wrong with the job which stops people from applying. It might be the job description itself, the conditions, or the salary. But surely it can't be particularly fruitful to just re-advertise the job again and again?