NEW YORK (Bloomberg) - Google Inc., the most popular Internet search engine, reported profit and sales that trailed analysts' estimates, signaling that an economic slowdown may be cutting into online advertising. The shares dropped 9.1 percent.

Fourth-quarter net income rose 17 percent to $1.21 billion, or $3.79 a share, from $1.03 billion, or $3.29 a share, a year earlier, the Mountain View, California-based company said yesterday.

That fell short of the $3.91 average analyst estimate in a Bloomberg survey.

Google's results, following a disappointing forecast from rival Yahoo! Inc. earlier this week, suggest that web use may be slowing as the economy cools. That may reduce demand for online advertising, the companies' main source of revenue. The number of US Internet queries dropped 3.9 percent last month, according to research firm ComScore Inc.

"There wasn't a lot of traffic," said Jane Snorek, who helps manage more than $70 billion in assets at First American Funds in Minneapolis. "December was a major disappointment.

"It seems like the consumer fell off the cliff."

Google shares, which have dropped 18 percent this year, fell $51.60 to $512.70 in extended trading after the report. They had closed at $564.30 on the Nasdaq Stock Market.

Sales, excluding revenue passed on to partner sites, climbed 52 percent to $3.39 billion.

Analysts predicted $3.45 billion on average in the Bloomberg survey. Excluding stock-based compensation costs, profit was $4.43 a share, missing the $4.45 estimate of analysts.

Yesterday's results were only the third time Google has missed analysts' profit estimates in its 14 quarters as a publicly traded company. The stock hit a record $741.79 on November 6.

Rising energy costs and a housing slump pushed down consumer confidence to almost a two-year low in January, according to the Conference Board.

Google investors are concerned that customers may respond by cutting their ad budgets. January marked Google's biggest one-month drop ever.

"Advertising-driven businesses, even if they're on the Internet, are typically affected by a slowdown, so Google is susceptible," Clayton Moran, an analyst at Boca Raton, Florida-based Stanford Group Co., said in an interview with Bloomberg Television.

He has a hold rating on the shares. "Caution is in order here."

Google increased its workforce to 16,805 employees in the fourth quarter, up 5.6 percent from the previous period.

The company's share of the US Internet search market rose to 56.3 percent in December from 50.8 percent at the end of 2006, while Yahoo's share fell, according to Nielsen Online.

Google accounted for 75 percent of US search advertising in 2007, up from 60 percent in 2006, according to New York-based research firm EMarketer Inc.

The company also is seeking to bring advertisers to its YouTube video site.

Google began letting marketers buy ads in YouTube clips last year. It also introduced software that allows sites to run videos and related ads alongside news articles.

This month, YouTube opened up its complete library of homemade clips and television shows to mobile-phone users.

Google also started selling ads on websites designed for mobile phones.

As part of its push into wireless devices, Google succeeded today in forcing the winner of airwaves being sold by the US government to open its network to any mobile device.

Bids surpassed the $4.6 billion threshold that triggered so-called open access rules, the Federal Communications Commission said, without revealing which companies were involved.