Microsoft Bing's U.S. September search share fell to 8.5 percent from 9.6 percent in August, with Google gaining more than two percentage points, according to analytics firm StatCounter.

Google's September share rose to 80 percent from 77.8 percent in July, reversing a trend of Bing gaining share at the expense of search engine rivals. Bing dropped more than one percentage point, the first sign that Bing might be susceptible to weakness since it launched in June, StatCounter found in stats released Oct. 1.

"The trend has been downwards for Bing since mid-August," said StatCounter CEO Aodhan Cullen. "The wheels haven't fallen off but the underlying trend must be a little worrying for Microsoft." See StatCounter's chart here.

The findings clash with estimates from Nielsen, comScore and HitWise, all of whom found that Bing continued to post gains at the expense of Google, Yahoo or AOL in August. It will be a few weeks before those research firms post their September statistics.

Yahoo, with whom Microsoft is hoping to partner with in search to close the gap on Google, also declined, to 9.4 in September from 10.5 percent in August.

StatCounter also said global search share for Bing and Yahoo also declined. Bing slipped to 3.3 percent from 3.6 percent, while Yahoo dipped to 4.4 percent from 4.8 percent. Google's global share remained at 90.5 percent.

StatCounter, which checks 10 billion page loads per month, said its data is based on an analysis of 4.6 billion search engine referring clicks collected from September 2008 to September 2009. The firm said 1.1 billion of those clicks hailed from the United States.

No one would presume to declare Bing a failure for slipping in one month. In fact, this fall was expected by many search analysts who believed Bing would see traffic surges and market share gains from users doing less searches on Google or Yahoo to test Bing. StatCounter's new stats may simply be a sign that the honeymoon is over, and that users, their curiosities satisfied, are returning to Google.

Bing will have to redouble its efforts, both in innovation and marketing (it has embarked on a $100 million ad campaign), to peck away at Google. The partnership with Yahoo, in which Bing will power Yahoo search results, should enable Bing to narrow the market share gap.

Microsoft, whose combined market share with Yahoo would be close to 30 percent, would still significantly lag Google and its 65 percent-plus U.S. market share. Google nets closer to 70 percent of the searches on a global basis, according to comScore.