I don't like the space for one major reason (and job hunting is isomorphic with dating): to optimize making money you have to fail to deliver on the expectations of those for whom you're matchmaking.

With dating/jobhunting if you create a supremely efficient market place that finds the match almost immediately your customers go away. They don't need your service any more. It behooves you to increase the inefficiency of your marketplace to deliver almost-matches that keep people interested and paying, but not quite good enough to make them leave.

Perhaps you could argue that a job board is different because employers are repeat finders and will come back again and again - the adultfriendfinder approach, people who keep getting laid keep paying.

Either way they're tough businesses to make money on AND deliver a great service.

LinkedIn does a ton of other things of course, to monetize, and isn't just a job board. But that just raises questions about some of their other practises of holding your own data hostage - which seems to be a key business model for social network monetization at the moment, and that's a whole other question.