Relevant news topic to this discussion:
http://www.ere.net/2013/08/01/linkedin- ... s-monster/
LinkedIn has passed Monster.com for recruitment-related revenue.
It's hard to imagine a future for companies like Monster at the moment, there's just so much more value-add for both the recruiter and the person being recruited on a platform like LinkedIn.
What I do think there is room are some services, sites, or networks that specialize in vocational roles, especially if they can pre-screen employees for certain kinds of skills (like LinkedIn Endorsements - but actually meaningful)
Edit: Just to add my own experiences to the body of knowledge we're collecting here: I recently took a new job (exec-level) that I learned about from one of the weekly "Jobs you might be interested in" spam emails from LinkedIn. It's the first time since my very first job out of uni 15 years ago that a job didn't come from a personal/business connection somehow.