NotchUp, an employment site with a novel twist, is throwing open the doors of its job marketplace to any company willing to pay applicants for the privilege of interviewing them. That approach raised some eyebrows when NotchUp launched its invitation-only test earlier this year, but it makes sense for some jobs, and the company is making the site more useful with new features like Facebook integration.

Unlike established job sites like Monster.com, NotchUp isn’t really focused on active job seekers. Instead, it helps employers attract people who are satisfied with their current positions but could still be tempted away at the right price (which is where the interview fee adds a little incentive). Users interact with NotchUp differently, too — rather than scouring the listings for potential employment, you just put your resume up on the site, set the price you want to receive for an interview, then get on with your life and work. Weeks, months or even years later an employer might think you’re a good fit for an opening and reach out through the site..

The Palo Alto, Calif. company’s new compatibility with social networking site Facebook, as well as with the cross-platform social network initiative OpenSocial, should make this process even easier. If users install the NotchUp application for Facebook or OpenSocial, they won’t even need to visit NotchUp to update their resumes; instead, all updates to your work history on Facebook or OpenSocial will be automatically updated on NotchUp. This could be a big draw for people who are interested in the site, but don’t want to put a lot of work into it.

The other new feature introduced today is the creation of a personal identification number, or PIN, for each NotchUp user. Security and privacy are still obvious concerns — for example, I wouldn’t want anyone at VentureBeat to see my profile on NotchUp, particularly if that profile said I was very interested in another job. (Even though recruiters can’t seem my name until I accept an interview offer, my current employer may still be able to figure it out from the details.) NotchUp already allows users to say which companies can and can’t see their profiles, but the PIN offers another way to control access — if you get a cold call from a recruiter, instead of dealing with them directly, you can just give them you PIN, which will allow recruiters to contact you through your NotchUp account.

All of this should make the site more useful for applicants, but are there employers actually willing to buy into NotchUp’s model? Apparently, the answer is yes — the company says quite a few interviews have been scheduled and paid for while NotchUp was in testing mode, and there were more than 1,000 employers interested in participating once NotchUp opened its doors.

It’s important to remember that NotchUp isn’t really aiming to help companies fill entry-level positions, where Monster.com and Craigslist do just fine, or top-level executive positions, either, which are usually filled by professional reruiters. Instead, NotchUp is aiming for the middle of the job “triangle” — namely, managers and other positions where you want someone who is well-educated, experienced and otherwise qualified, but may not want to pay the fee for a recruiter.

It’s too early to say whether NotchUp will be successful, but at least it’s taking a smart (and entertaining) approach.