We always like to read the yearly study that Mark and Gerry at CareerXroads put together on hiring: CAREERXROADS 5TH Annual Sources of Hire Survey.

As its outlined, the study is only based on 24 companies, so it can only be seen as anecdotal, but we found some interesting data to consider.
18% of the labor force is contingent (temporary or contractor).
The average requisition load is 140 requisitions per year per recruiter.

We summarized their findings of hires by source in the chart below: