SAN FRANCISCO --

Google

(GOOG) - Get Report

and

Yahoo!

(YHOO)

built their entire fortunes on the promise of a global move into online advertising.

But a sluggish economy has exposed the vulnerability of that very revenue stream -- and raised some doubt as to whether the growth levels the industry once enjoyed will ever be realized again.

Most observers agree that the dwindling wealth now seen in online advertising is a temporary byproduct of the housing crisis and high gasoline prices, which have curbed consumer spending as well as marketing budgets. Internet display ads, especially, have been hard hit as the concept of branding seems more like a luxury than a necessity when money is tight.

An economic recovery, however, provides no guarantee that revenue from online advertising will reach its former heights. Sanford Bernstein analyst Jeffrey Lindsay, for example, offers an optimistic scenario of annual online ad growth of 12% four years from now, which is still well below the 40% growth in global revenue the industry saw in 2006.

His pessimistic scenario, which assumes a deepening housing crisis, has year-over-year growth slowing to 10% in four years.

Nonetheless, Lindsay remains upbeat about online advertising and expects to see high and persistent growth rates online despite a significant slowdown of businesses' total advertising spend, which also accounts for traditional mediums like newspapers and TV.

"It is the switching of advertising dollars from off line to online that continues to give the online sector such high resilience," he wrote in a recent report.

Indeed, for all the decelerating growth in online ads, the overall ad industry is expanding even more slowly -- precisely because of the shift away from traditional media. In his pessimistic scenario, Lindsay estimates only a 4.3% rise in total advertising revenue growth this year vs. a 20% growth for online ads alone.

But Stanford analyst Clay Moran advises caution on online advertising, noting a deceleration in e-commerce spending, which in turn influences the way businesses manage their marketing dollars.

But even after emerging from a recession, things may never be the same. "From a big picture, it's hard to imagine that the growth doesn't slow as the industry matures," Moran says.

He adds that most investors realize that growth rates can't remain extraordinarily high forever. Indeed, even a superstar like Google is finally starting to see some weakness, although it's unclear how much of it has to do with the current economic climate.

The company took a beating last month when it missed Wall Street estimates. And although revenue was up 39% over last year, it was up only 3% sequentially. Moran expects Google's full-year revenue to grow by 39%, a significant drop from 60% last year. For next year, he expects revenue growth of 27%.

Yahoo! showed even weaker revenue growth in the second quarter, up a mere 6% from a year ago. Its softening performance with display ads reflected a larger industry trend, with sectors like travel, retail and finance feeling the most pain. Moran's expectations for Yahoo!'s revenue growth is far lower than for Google -- about 11% this year compared to last year's climb of 12%.

Shar VanBoskirk, an analyst for Forrester Research, blames Yahoo!'s problems squarely on the company itself and not on any trouble facing the online advertising industry, which she maintains is still very robust.

"Right now, as an advertiser, it's exhausting to go to Yahoo!" she says, noting that the company is still trying to align all its recent acquisitions so they can function more seamlessly.

VanBoskirk acknowledges a slowdown in certain sectors of online advertising, like display. But she sees plenty more potential in the future. She points out that while many large companies have moved to online advertising, there is still a lot of medium and small businesses not there yet, leaving wide open a huge untapped resource.

Moran also notes that the online advertising industry is constantly looking for more revenue streams in the event that the old ones eventually dry up. Although online search and display ads make up the vast majority of what makes money on the Internet, there has been an effort for companies like Yahoo! and Google to diversify through mobile ads and video. Companies are also trying to realize more efficiencies from their existing technology.

Still, it's hard to say whether all of that will be enough to make up what someday might be lost.

"Video and mobile are extremely small," Moran says. "As an investor, I wouldn't count on that because we don't have concrete evidence that they will be material drivers. There's still some time before that becomes concrete."