We’re inclined to see the internet, old as it now is, as still the baby of all media, growing at a gangbusters pace but from a small base.

It is indeed, but that’s about to change, predicts a new forecast for global ad spending.

The gangbusters pace will continue, but the base is getting larger by the day, and within a short few years the internet could well become the world’s fourth-largest medium, behind only television, newspapers and magazines.

Next year, ZenithOptimedia predicts the internet will surpass outdoor advertising, and in a few years after that it will move past radio.

Just when it will pass radio is not entirely clear, but it seems destined to happen, says Jonathan Barnard, head of publications at ZenithOptimedia, which has just released its global spending forecast for the years through 2008.

“We don’t think that the internet will overtake radio in our forecast period,” says Barnard. “But it will be roughly the same size as radio and catching up fast. It may well overtake within a few years after our forecast period.”

The company’s forecast shows that globally, between 2005 and 2008, internet advertising expenditure will grow a total of 72 percent. That’s a 19.9 percent annual average growth rate.

“The internet is growing three times faster than the advertising market as a whole,” says Barnard. Overall global ad spending is forecast to grow at 5.6 percent a year on average.

Though outdoor is expected to grow at a faster pace, at about 6.8 percent per year for the period, by next year it will account for 5.5 percent of total global advertising expenditures, while the internet will claim 6.1 percent, up from 4.5 percent in 2005.

By 2008 the internet is expected to account for 6.5 percent of all global advertising expenditure.

Radio, meanwhile, is growing at a much slower annual rate of about 3.0 percent, and as it does its share of spending is slipping. From 8.5 percent in 2005, ZenithOptimedia expects radio to fall to 7.9 percent by 2008.

But even at its fast pace, the internet will still lag far behind the next-largest category, magazines. While global advertising expenditure on the internet is forecast to be $31 billion in 2008, magazines are forecast to weigh in at $61 billion.

No surprise, the internet is growing fastest in some of the fast-growing developing countries where it is not well established at this point.

The fastest-growing internet market is forecast to be Russia, where advertising spending on the medium is expected to shoot up a total of 405 percent between 2005 and 2008. Poland ranks second with growth forecast at 345 percent.

Yet, even in the U.S. and Britain, two countries where the medium is well established, internet spending is forecast to hit 57 percent and 86 percent total for the period 2005 to 2008.

The forecast also shows that despite the rampant growth of the internet during the forecast period, there are signs of a slight slowdown.

Globally, ZenithOptimedia says that internet advertising spending grew 26 percent in 2005. The company forecasts that figure will grow 29 percent in 2006, 18 percent in 2007 and 13 percent in 2008.

“It has to slow down really, because it has been growing so rapidly. In part this is just a function of the scale. It is getting so large. But it is still adding plenty of dollars,” says Barnard.

Meanwhile, elsewhere on the internet during the week ended April 2, Microsoft, Yahoo, Time Warner, Google and eBay took their customary spots as the top five parent companies, according to Nielsen//NetRatings.

GUS Plc was the top advertiser for the seventh straight week, with Vonage, Netflix, Classes USA and United Online right behind. Verizon fell out of the top five, from No. 3 to No. 7.

The top five advertising sites during the week were Yahoo, MySpace, MSN, AOL.com and Juno. That knocked iWon.com out of the top five, from No. 5 to No. 10.

Average computer sessions per person remained even at 16, though average domains visited per person fell 2.6 percent from 39 to 38. Average PC time per person was also down about 2.8 percent, from 16 hours and 56 minutes to 16 hours an 27 minutes.