Job websites on the internet have been around for years, but they have severe limitations.

Recruiters can find hundreds of jobseekers relatively quickly for a given position, but a lot harder time sifting through them for quality. More recently traditional jobs websites catering for exclusive clients have appeared, like TheLadders. com sponsored by the Wall Street Journal and Business Week, which only advertises positions that pay $100,000 per year or more.

But they still have similar drawbacks, not the least of which is that there is a level of commitment required to get people to post a CV . . .including the willingness to risk that their current employer will find out.

"One of the biggest advantages of LinkedIn is that it's not just seen as a job thing, " said Sarah McKenna, a Dublin-based recruiter with Google who has watched LinkedIn explode in its usefulness.

Many people start out by using it to find former office mates. "It has the benefit of targeting people in a nonthreatening way."

To see how it works I signed up for a LinkedIn account. Using its search engine I discovered within minutes that several excolleagues were members, in Dublin, New York, London, Paris and Chicago.

I also quickly discovered profiles belonging to CEOs and senior executives at some of the world's biggest and best-known companies.

Not all, perhaps not even most, were looking for a new job. Many simply heard about it by word of mouth as a networking tool to help gin up new business prospects. Because LinkedIn has grown so quickly in recent months, a 'network effect' has started to take hold. The network becomes more effective because it has reached critical mass . . . more people joining the site.

Andrew Smith is cofounder of Object Marketing, a marketing consultancy for technology companies based in Shepherd's Bush in London.

He was a colleague of mine at another UK firm six years ago with whom I'd lost touch and one of the first people I found on LinkedIn. He's been a member for perhaps two years but isn't looking for a new job.

"I get maybe one or two contacts a week via LinkedIn, sometimes from recruiters asking, 'Do you know anyone who might be good for this job?'" The thing about LinkedIn is that Andrew may be proving far more useful than he realises to recruiters, because he's already sharing his contacts book with other people in his own 'connections'.

A recruiter with whom Andrew has a relationship will be able to search not just his own contacts, but Andrew's and as many others with whom the recruiter has cemented an online relationship.

Using the search engine to look for a business development manager, for example, the recruiter may discover that Andrew knows three or four other people who might be suitable. The recruiter wouldn't necessarily be able to see the candidate's name or email address . . .unless they've chosen to make it visible to everybody . . . so they ask Andrew if he'll make an online introduction. The prospective candidate's name is still protected until and unless she chooses to reveal it to the recruiter.

But while some are undoubtedly using it strictly to find new job opportunities, many are on LinkedIn or competing sites like Ryze or Spoke to find long-lost work buddies.

"I spent a number of years abroad and have found it useful to contact ex-colleagues and contacts, " says Michelle Noone, a recruiter with KPMG in Dublin. She's used it primarily to link up with former colleagues in the UK and the US but thinks it's catching on in Ireland.

Sarah McKenna agrees.

She also sees another benefit for recruiters: because you're publishing your work history for all to see, you are less likely to exaggerate than on a CV, which you might imagine will only be seen by your prospective employer.

"People tend to be an awful lot more candid with their profile than on a CV. If I see someone quoted in an article claiming to have delivered 4bn in revenue and then find a different number on their LinkedIn profile claiming a different number for the same job, it raises an eyebrow."

LinkedIn isn't an entirely free service.

While you can post a profile and be found for free, to use most of the advanced features you could pay around $200 for a year. You can also purchase, for $10 each, a service whereby the website will send an introduction on your behalf to someone with whom you don't share a connection.

LinkedIn claims that 25% of members respond to the online version of a cold call.

Better-known social networking websites MySpace and Facebook, seeing the success of LinkedIn, have added basic job-search functions in recent months. That may prove to be more vigorous competition in the future, but for the moment the youthful reputation of other sites will probably deter older (and usually more valuable) executives from joining those sites.

For the moment, LinkedIn has a lucrative . . .

unlike most social networking sites it is cashflow positive . . . field almost to itself.