Bloomberg: Why Employers Are Slow to Fill Jobs: Business Class (h/t: Greg Mankiw):
Employers influence the pace of job filling by the intensity of their recruiting efforts. Under an aggressive approach, an employer places help-wanted advertisements, reaches out to attractive candidates and retains professional recruiters. Under a relaxed approach, an employer tends to rely on word of mouth or waits passively for applicants. Employers can also screen candidates quickly or slowly. They can set a high or low bar for experience, qualifications and promise. They can offer more or less attractive compensation packages.
Steven J. Davis goes on to explain the range of reasons why the recruiting intensity has diminished.