Google (NASDAQ:GOOG) co-founder Sergey Brin said in May that a monetization deal with News Corp.'s MySpace "didn't pan out as well as we had hoped." Whether he realized it at the time or not, he was foreshadowing darker days for social networks.

Last week, researcher eMarketer lowered its 2009 spending estimate for social networking advertising to $1.3 billion, from $1.8 billion. "While hardly surprising given estimate cuts for the broader online advertising industry, the news is a sobering reminder of the need to find business models that extend beyond advertising," wrote Mashable's Adam Ostrow in reporting the news.

Facebook, Facebook, where did your growth go?
He's right. Ad buyers are finally tapping the brakes on digital ad spending, even as they abandon more traditional media, such as New York Times (NYSE:NYT), Viacom (NYSE:VIA-B), and NBC Universal, which is majority-owned by General Electric (NYSE:GE).

But doing the math tells me that Facebook could be under more pressure than most. Details on that in a minute. First, let's review the latest market-share data from Web-traffic tracker Hitwise:

Social Network

Market Share

MySpace

67.54%

Facebook

20.56%

myYearbook

1.65%

Tagged

1.53%

Bebo

0.94%

Source: Hitwise. Data current as of August 2008.

At the time, Facebook was on track to grow 50% year over year. Today, its user population is more than twice the size of France. So let's assume that Facebook added another 10% to its share as of the end of August.

Now for the math. Assuming that eMarketer is correct, and there's $1.3 billion to be spent on digital ads next year, Facebook, with 30% of the market, would lay claim to $390 million in ad revenue. That's assuming no other middlemen, fees, etc., which usually isn't the case.

Maybe Facebook will do better than that. Maybe it'll eat market share like an NFL lineman eats Chunky Soup. Or maybe CEO Mark Zuckerberg's monopoly-money bet for social superstar Twitter was an attempt to boost growth in a slowing market.

Forgive me if I suspect the latter. Earlier this year, Zuckerberg told reporters that Facebook would generate $300 million to $350 million in revenue, doubling 2007's total. Makes $390 million seem sort of paltry, doesn't it?

Facebook is a useful service that's growing massively. It'd be pure folly to suggest that Zuckerberg and team can't produce at least $600 million in 2009 sales. But with the ad-revenue pie shrinking as it has, that target will be a lot harder to reach than most thought it would be even a month ago.

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Fool contributor Tim Beyers is a member of the Rule Breakers team and had stock and options positions in Google at the time of publication. Check out his portfolio holdings and Foolish writings, or connect with him on Twitter as @milehighfool. The Motley Fool is also on Twitter as @TheMotleyFool. Its disclosure policy dreams of a coveted spot on the Fool's Facebook page.