The warning prompted financial company Merrill Lynch on Friday to lower its estimate of Google's sequential net revenue growth to 7 percent. "Taking seasonality warnings to heart, we revised our estimates," stated the report. Last quarter, Google's net revenues grew by 12 percent.

The predicted summer slowdown in growth raises the question of whether seasonal trends apply equally to search marketing as compared to other types of advertising. Advertising on TV, for instance, falls in the summer when fewer people are watching--either because they're spending more time outdoors, or simply cutting back on TV because the networks are showing re-runs.

An OnlineMediaDaily analysis of 2004 ad spending estimates from TNS Media Intelligence/CMR shows that just 18.1 percent of the total year's network TV ad spend occurred during the third quarter, compared to 22.4 percent in the first quarter, 23.5 percent in the second quarter, and 35.9 percent in the fourth.

In 2004, the only year for which data is available, Google's second and third quarters saw slower growth than the rest of the year. At the beginning and end of last year, quarter-over-quarter revenues grew by around 28 percent, compared with 7 percent in the second quarter and 15 percent in the third--a jump that Reyes speculated might have resulted from the well-publicized initial stock offering.

eMarketer Senior Analyst David Hallerman says that the relative weakness in search advertising during the second and third quarters likely stems from a slowdown in growth on the consumer side, as opposed to the advertiser side. In the summer, apparently, people aren't increasing the time they spend performing searches--at least not at the same rate as when the weather is bad.

What's more, he adds, compared to TV, advertisers don't have to rely on estimates of how many people will view their ads. Instead, they set their per-click price, and then can sit back and wait to see what will happen. "In search, nobody gets any money unless the user clicks," Hallerman said. "All other forms of advertising, it's just the buyer and seller. The audience has no choice in the matter."

This news story is important. I recommend that everyone read it.