To arrive at a value proposition I dare say you really have understand your customer and their customer and so on.
As you do research you will notice all the job sites connect unique employer\employee combinations, situations, geography. For E.G
http://www.standoutjobs.com started matching tech talent with funded startups now their value prop is a platform for integrating with employers sites.
http://www.hippo.com.au helps student find casual\part-time work at McDonalds, KFC, local cafe.
http://www.peopleahead.com/ automatically provides job opportunities to employed and experience professionals. There value prop is a true match system that drops job opportunities into their inbox without lifting a finger
http://www.adultstaffing.com OBVIOUSLY connects people in the adult film industry.
http://www.gulftalent.com matches professionals with companies in the united emirates.
Basically, know thy customer, know thy industry, know thy weakness. Know beyond reasonable doubt who in the room will fall in love and why then go to work making the how.
Closing off with two tips from friends in the start-up recruitment game. Is to have a contingency plan in preparation for getting sued by the big co's when you gain traction. And pressing the flesh creating real "physcial" relationships (Not SEO) is important to getting clients who will be stayers and help improve the service.