This is what I remember from career training class about job advertisements:
As someone already said, only about two thirds of jobs are "advertised". The reason for this is, it costs money/time to do this. If you put it in the paper, you pay for the ad. If you add it to the website, you have to worry about when to take it down and must be ready to receive electronic resumes etc....
Back in the day where the hospitals were hiring like mad, they'd overlook those inconveniences. They don't have to anymore not though. For every person who limits themselves to using the paper/internet alone for job searches, there are ten or more who are hitting the streets and applying everywhere. They no longer need to reach out to us, it is us reaching out to them.
The class then went on to debate that using the paper/internet was probably the worst way to do a job search
because, well, the jobs that are advertised are the ones they can't fill without doing so. Its an assumption yes, but a safe one, that the ones being advertised are not getting filled for a reason. The better jobs are not advertised.