Companies have long encouraged employees to recruit friends and former colleagues to help fill job openings, often rewarding them "referral bonuses" of hundreds or even thousands of dollars if those applicants are hired.

Today, thanks to LinkedIn, Facebook, and other social-networking sites, the technological foundation exists to take the referral process to a whole new level: Why call just one friend when you could alert many people to the availability of a plum position?

A company called Zao, in fact, offers a platform for doing exactly that. You can filter your personal and professional networks by skills, experience, and other criteria and then alert those who make the cut to a potential job.

It sounds promising, but to date the nascent phenomenon of "social recruiting" seems to be a one-way street: Job-seekers are working their networks hard, dropping a note to let a friend know that they have applied, or are interested in applying for, a position at the friend's company, for example. Employees don't seem to be helping their employers much by using their networks to spread the word or round up a posse of viable candidates.

Dave Zielinski, writing for the Society for Human Resource Management, notes that a survey by Jobs2Web (a recruiting technology company that's part of software maker SuccessFactors) found that only 4 percent of client employees have made more than one referral via their social networks.

Employees may simply be too busy, but their endorsement of a candidate is so valuable that some companies are getting creative about how to pull them into the process via social networking. Accenture, for example, now includes a Get Referred button on the jobs it posts on its website. When an applicant clicks the button, they can see whether any of their personal contacts work at Accenture; if so, they can ask the Accenture employee to make a referral on their behalf.

Troy Henikoff, CEO of Excelerate Labs (who previously led several other high-tech start-ups), is a big fan of referrals. "It's amazing, despite the many places you can post job openings, how many people come in through referrals or recommendations," he says. "But it makes sense, because the things that are critical in a job, like trust and reliability, can't really be captured in an ad. So, you need to let your network know that you're looking for people, because they can't help you if you don't know."

And you may have to rethink that no-Facebook-at-work policy, because it may hamper your efforts in the war for talent.