The big story about job ads is how much they've moved onto the internet from newspapers, but there's a far bigger story looming out there, and it's the huge growth that's expected in online recruitment advertising in the coming several years.

And what will drive it is not something that would immediately leap to mind: baby boomers.

The first wave of boomers are now hitting retirement age, and as more and more collect their gold watches and head for the golf course, it will create a huge talent void in the workplace that employers will have to fill.

What will make it especially tough is that the next generation down, Gen X, is a particularly small one, too small to meet those needs.

"What we will find is that there is going to be an increased need for human resources to be actively recruiting constantly because there is higher turnover and more openings,” says Peter Conti, senior vice president at Borrell Associates, a media research company specializing in local internet advertising.

The result will be a bustling recruitment market, predicts Borrell in a new study, with total recruitment spending growing 25 percent between 2008 and 2012, to $73 billion.

The increase in demand for workers is already kicking in. Borrell quotes a study from Hyrian, a large recruitment process outsourcing company, which found that last year there were about 4.5 million more jobs than people. That figure is about 5 million this year and is expected to be about 10 million by 2010.

The big winner as this demand picks up will be the internet, which will see recruitment advertising rise 43.5 percent, to more than $11.5 billion.

Within online recruitment, Borrell expects the biggest growth will be among the smaller, more targeted job boards.

“The general job boards, such as Monster and HotJobs, are not satisfying the recruiters,” says Conti.

That’s because of the vast number of resumes, many unqualified, that ads on these sites can generate. The employer is left with the daunting task of sorting through to find the much smaller number who do meet the job qualifications. The big sites are working to rectify this by developing tools to help filter the resumes, but that could take some time.

Another big gainer will be traditional full-service employment agencies and temporary agencies. Even with the rise of the internet they've continued to be a huge force in recruitment spending, together accounting for 76.4 percent of total spending. They've maintained share by providing that sorting-through process the internet can't.