Some day, perhaps sooner that folks think, the sizzling growth in internet ad spending will begin to cool down.

But that slowdown is nowhere in evidence in the latest data from the Interactive Advertising Bureau for full-year 2006 internet ad spending and the final fourth quarter of that year.

According to the IAB, which released the data yesterday in conjunction with PricewaterhouseCoopers, online ad spending for the year rose 34 percent, to $16.8 billion, topping both 2005 and 2004, and was more than double the spending in 2003.

Spending in the fourth quarter grew 32 percent over the year-earlier period, to $4.8 billion, making it the fastest-growing quarter in a decade. Further, it was up 15 percent over the third quarter, with spending of nearly $4.2 billion.

Through the year, each quarter in fact showed decent to strong gains over the prior period. Third quarter was up 2 percent over the second, but the second was up 5.5 percent over the first, and the first was up 8 percent over fourth-quarter 2005. (See chart, below.)

The IAB data is not final. The final figures will be released next month, but they certainly show the robustness of the internet at a time when other media, such as magazines, continue to struggle against flat or modest growth in ad spending.

The trends behind the 2006 growth have been in place for some time. More mainstream advertisers are shifting more dollars to the internet and out of traditional media, attracted in large part by the internet's greater accountability but also by the ever-rising numbers of Americans who are spending more time online.

But certainly another factor is that an increasing number of traditional media outlets that are launching offshoots, or building up existing sites to offer more functions, then selling advertising on those sites as they go out to sell their existing media offerings, whether TV or radio time or newspaper print ads.

For years, people have talked about the time when the pace of online spending would slow, and just recently eMarketer put out a reporting saying it would be in 2007.

That forecast has internet ad spending slowing this year to a pace of 18.9 percent (see chart, below), building back up to 22.1 percent in 2008, then again dipping for several years thereafter.

That reflects two things. One is that the growth percentages naturally begin to fall as the base of the spending gains mass. A dollar added to another dollar represents a growth of 100 percent, but that same dollar added to $5 represents just 20 percent growth.

And the base of online spending more than doubled between 2003 and 2006.

But the other factor in the projected slowdown, according to eMarketer, is the U.S. economy, which is weakening.

"Between the budget deficit and the trade deficit and softness in the U.S. dollar, you can expect some softness in the economy," David Hallerman, senior analyst at eMarketer, told Media Life in a recent interview. And that softness will work its way across the ad economy, eventually dampening spending on the internet.