It used to be that the "who you know" would be a recruiter. They used to be the ones who knew the inside track. Nowadays, the turnover rate is so high that there isn't any time for them to establish a relationship with their peers in the companies hiring.

It also used to be that they knew how to identify the right candidates for a slot. I agree with the comments that they know less about IT sometimes than the Hr rep at the company doing the hiring. It's a shame, but all recruiters do these days is weed out candidates. That doesn't always mean weeding out the right ones, however.