This is actually a really great question, although you might be disappointed with my answer. As a Headhunter/Recruiter with an Engineering degree who has worked for some of the most exciting startups in the world, as well as Fortune 100 companies, I have found that job-boards are rarely successful in providing strong candidates. Now, I will also say that sometimes they do come through with 'flying colors', but that is more the exception than the rule.
I personally use ZipRecruiter, which posts my positions to over 100 sites, and sometimes it does work. Usually not so well, but once in a while I get a really outstanding candidate that way. Because this site posts to so many other sites, I'm pretty certain that it reflects the efficacy of job-boards quite a lot. I think that it's an inescapable conclusion that job-boards don't work nearly as well as direct-contact via email or phone.
I will post job openings on Twitter, LinkedIn, GooglePlus, and Industry-specific society sites like AIChE, ASME, SPIE, ACS, IEEE, etc. Generally, these do not work nearly as well as having a skilled recruiter directly sourcing and contacting individuals with details about a job opportunity.
I've been doing this for 27 years, and it's always been that way. I've certainly posted ads on Monster.com
, too, but rarely got any results. ZipRecruiter is probably the best bargain out there, and it's still a good tool to have, but direct-contact marketing by a Headhunter is 90% more effective than posting to even 100 job boards at the same time.
So, based on some feedback, here, I want to clarify that I only use job-boards as a back-up or "last resort". They rarely produce good results, but the fact is that they can -- and it would be foolish to ignore their possible benefit... That said, even posting on 100 job boards, only a very small percentage of the candidates I identify come from that source.