How do job seekers find that "hidden" job.

I hear it said so often that most jobs are not posted on job boards or organizational career pages therefore networking is so key to finding that "hidden" job or have a job created just for the person. my HR background says this smells. This may be true for small companies but I find it hard to believe "most" jobs are not posted in some manner somewhere. Is this a fallacy or over-generalization for the job seeker?