Not that there aren't good third-party recruiters out there, but the majority I've encountered while personally looking for work were disappointing. They seem to scrape resumes from the Internet, perform rudimentary keyword-matching, and cold-call for candidate volume rather than candidate quality. Part of the issue is that it's in their best interests not to find a best fit for either party, but rather to maximize their own overall throughput. If you decide to use a third-party recruiter, then please, for your own sake and those of your potential hires, vet them thoroughly.
Yeah, that's the feeling I've gotten. There seem to be a group of recruiters, and a group of serial contractors, that cycle around SV. In this market, it's almost just a matter of manpower and positive outreach. I guess that's what dev advocates are...
