Microsoft attracted the largest number of website visitors worldwide in December, a recurring accomplishment that makes the company's disappointing online ad business all the more disconcerting.

Close to 509 million unique visitors, ages 15 and up, went to Microsoft websites in December, according to comScore Networks. Google came second with a little over 494 million, while Yahoo took third place with almost 477 million unique visitors.

Microsoft also led in March 2006, when its lead over Google was bigger. Microsoft had almost 539 million unique visitors to almost 496 million for Google. Third place went to Yahoo with little over 480 million.

However, the difference in online ad revenue between Microsoft and its two rivals is massive, a sign that, despite heavy investments in the past three years, Microsoft isn't close to matching their ability to monetise website traffic, especially Google's torrid growth in the search engine space.

In the quarter ending 30 September 2006, Google's revenue, which comes mostly from search engine ads, grew 70 percent to $2.69bn, while Yahoo's revenue, mostly derived from various forms of online ads, came in at $1.58bn, up 19 percent. By comparison, in that same quarter, Microsoft's Online Services Business, which includes online ad and internet access sales, generated only $539m in revenue, down from $564m in the same quarter the previous year.

Microsoft executives have acknowledged that the Achilles heel of its online ad business is its search-engine segment. In December, Microsoft's share of US search engine queries continued to fall, landing at 10.5 percent, a distant third place from leader Google, which nabbed 47.3 percent, comScore said in a separate study. In July 2005, Microsoft's US search engine query share stood at 15.5 percent and Google's at 36.5 percent, according to comScore.

Overall, unique website visitors grew 10 percent globally from December 2005 to December 2006 to almost 741 million. Microsoft and Yahoo both grew 5 percent each. However, Google topped the market growth with a 13 percent increase, in part due to its international appeal and to the rising popularity of some of its non-web search services, comScore said. For example, Google's image search engine grew its unique visitors by 40 percent, Gmail by 71 percent and Google Maps by 62 percent.