I've been surveying the strategies most small employers use to attract new employees. The process looks quite a bit like it did 5 or 10 years ago. Very few employers take into account where the best candidates actually are, and how to best reach them. So when candidate behavior changes, as it has ... most employers miss it because job postings follow tradition. Of course, when you ignore where the best candidates are, you doom your recruiting process from the start.
Most employers try to save time and money in one of two ways:
They try to select job advertising venues that yield a relatively small number of resumes--niche job boards for example. In theory, only "insiders" know about the job board, so "outsiders" apply less frequently and there are fewer bad resumes to wade through. The rationale is that it is worth paying a bit more for a job board if you don't have to review so many bad resumes.