WASHINGTON - Google said Wednesday that its fourth-quarter profit nearly tripled from a year ago as the world's most popular search engine continued to expand its dominant role on the Web, especially in the highly lucrative area of online advertising.

The company said its net income increased to $1.03 billion from $372.2 million in the fourth quarter of 2005. Revenue reached $3.21 billion, up 67 percent.

Chief executive Eric Schmidt attributed the company's success to its search engine, which is becoming increasingly well established beyond the English-speaking world.

"Search is getting better and better as we continue to innovate around the globe," he said in a conference call with analysts.

Search has become the main way that users navigate the Web. It has also become central to advertising that is tailored to match specific search requests, a form of ads that has revolutionized the marketing business.

The eight-year-old Mountain View, Calif., company has inexorably widened its lead over other search engines despite rivals' intensive efforts to improve their offerings. During the past two years, Google's share of U.S. searches expanded from 35 percent to 47 percent while Yahoo's slipped from 32 percent to less than 29 percent and Microsoft's fell from 16 percent to less 11 percent, according to ComScore Networks.

Google's stellar earnings contrasted with far less encouraging results last week from Yahoo and Microsoft. Yahoo reported a 61 percent drop in quarterly profit as it continued to fall further behind in the contest for search-related ad revenue.

Google's breathtaking growth wasn't enough to satisfy investors, who sold shares after the company's results were released.

In after market trading, Google's shares drooped $8.05, after having risen $7.18 to close at $501.50 on the Nasdaq Stock Market.

Wall Street's tepid reaction to the fourth-quarter performance illustrates the pressures facing Google as it tries to maintain enough moneymaking momentum to keep shareholders happy while investing heavily in new employees and data centers to support its ambitious plans for the future.

(The Associated Press contributed to this report.)