Oct. 14 (Bloomberg) -- Google Inc. and Microsoft Corp. boosted their share of the U.S. Internet search market in September, while Yahoo! Inc. lost ground, according to research firm ComScore Inc.

Google’s share increased to 64.9 percent from 64.6 percent in August, Reston, Virginia-based ComScore said yesterday. Microsoft rose to 9.4 percent from 9.3 percent, while Yahoo dropped to 18.8 percent from 19.3 percent, ComScore said.

Microsoft, which has increased its share for four straight months, announced an agreement with Yahoo in July to team up against market leader Google. Under the 10-year accord, Microsoft will gain more users for its Bing search engine, unveiled in June, and the two companies will share revenue for ads that appear next to query results. The companies expect to close the deal early next year.

“Bing continues to nibble away at Yahoo,” Youssef Squali, an analyst with Jefferies & Co. in New York, said in a research note today. “After losing market share in July and August following the much-heralded Bing’s launch, Google has climbed back up to the pre-Bing levels.”

Dana Lengkeek, a spokeswoman for Yahoo, said the focus should be on long-term trends and not month-to-month swings.

“We continue to invest in our search experience to drive innovation and user engagement,” Lengkeek said in an e-mail.

Google, based in Mountain View, California, rose $9.21 to $535.32 at 4 p.m. New York time on the Nasdaq Stock Market. Yahoo, based in Sunnyvale, California, added 7 cents to $16.95. Microsoft, in Redmond, Washington, climbed 15 cents to $25.96.

Facebook Inc., the No. 1 social-networking site, had 95.5 million U.S. users during September, up 3.6 percent from the previous month, according to ComScore. The number more than doubled from a year earlier.

Twitter Inc., the No. 3 social-networking site in the U.S., had little growth in September from the previous month, with 20.9 million users. Compared with a year earlier, though, its users have swelled 18-fold.

To contact the reporter on this story: Brian Womack in San Francisco at bwomack1@bloomberg.net

Last Updated: October 14, 2009 16:42 EDT