In the fall of 2008, when the worldwide economy began to melt, Google responded by shamelessly expanding ad coverage on its web-dominating search engine, letting more ads onto more pages. But now that the economy has recovered, the web giant has suddenly become more much vigilant in its efforts to weed out what it considers low-quality advertising.

According to new data from AdGooRoo - a search marketing consultant that tracks search ads from a network of servers across the globe - Google permanently banned 30,000 accounts from its AdWords ad system at the beginning of December. That's roughly 5.3 per cent of its active advertisers. Ad coverage dipped nearly 10 per cent in the wake of the mass axing, and yet AdGooRoo's data indicates that Google's revenues surged in the fourth quarter, thanks to increased competition for placement among the web's top ad spenders.

In other words, when the economy was in the tank, Google needed all the extra revenue it could get. But now that the economy is healthy again, it can step up efforts up to remove what it sees as inappropriate ads. The big boys are spending more, so it doesn't need as many clicks to boost the bottom line. What's more, by shrinking ad coverage, Google can actually drive more traffic to the big spenders. If you cut 30,000 advertisers, those still on the search engine get more clicks - and the more clicks, the more those big spenders pay.

Google announces its fourth quarter financials next week, and AdGooRoo data is typically a reliable indicator of what's to come. "Our ad coverage metric confirms that something big went down at the Googleplex last month. Ad coverage, which has been steadily climbing for the past 12 months took a sudden and precipitous dive in December," reads the firms latest search-ad report, due out on Monday. "Ordinarily, this would foreshadow a weak quarter, but we believe that this small drop will be more than offset by strong ad revenues."

Two years ago, in January 2008, Google famously began an effort to shrink ad coverage on the world's most popular search engine. This continued through the middle of 2008, and when the subject came up during Google's quarterly earnings call that July, senior vice president Jonathan Rosenberg attributed the shrinkage to Google's "continued focus on quality" advertising.

"[Google co-founder] Larry [Page] says we'd be better off showing just one ad [per page] - the perfect ad," Rosenberg said, indicating that coverage would continue to shrink.

But then he was interrupted by Google's other co-founder, Sergey Brin, who piped up with what can only be described as a shocking moment of candor. "There is some evidence that we've been a little bit more aggressive in decreasing coverage than we ought to have been," Brin said. "We've been reexamining some of that."

The economy was softening, and sure enough, coverage soon began to expand. According to AdGooRoo's numbers, Google's search engine showed 57 per cent more ads per page in the fourth quarter of 2008 than it did in Q3, as the economy imploded following the infamous demise of Wall Street stalwarts Lehman Brothers and Merrill Lynch.

Google had made significant changes to AdWords that fall, and naturally it said this would also improve ad "quality." So, whether Google is shrinking ad coverage or expanding it, the only aim to provide the world with better ads.

But surely it's obvious that Google is dialing up and down as the economy rises and falls. And now that the economy is on the rise again, the company has renewed efforts to crack down on ads it doesn't like. Google started banning advertisers en masse in early October - just as it was about to announce that the economic meltdown was over - and this house-cleaning came to head on December 3.

The official line is what you'd expect from the Mountain View Chocolate Factory. "Google is constantly working to ensure that we’re showing ads to our users that are relevant, in accordance with our ad policies, and safe for users. To that end, we perform regular reviews, using both manual and automated processes, in order to detect and disable ads that violate our policies," it told Search Engine Land.

But surely, the timing is no coincidence. ®