Microsoft isn't commenting on reports it may buy a stake in social networking site Facebook, but the company is touting gains it's made in the digital advertising market -- where it trails rivals Yahoo and Google -- thanks to a Web 2.0 acquisition that isn't just speculation.

Microsoft said Tuesday that more than 20 online publishers have switched to the Atlas ad platform it acquired through its $6 billion buy out of aQuantive, which was completed in August.

Microsoft said its newly formed Advertiser and Publisher Solutions Group, led by former aQuantive CEO Brian McAndrews, has recently won accounts from SmartBrief.com, Reunion.com, Entrepreneur.com, and seventeen other publishers.

The Atlas platform, which was operated by aQuantive's Razorfish unit, helps publishers create, market, and sell their digital ad space. The platform also includes an assortment of tools aimed at advertisers and agencies.

The $6 billion that Microsoft paid for Atlas and other aQuantive properties is an indication of the urgency behind the company's digital advertising push. In the fiscal year ended in July, Microsoft reported $2.47 billion in revenue in online services, including advertising -- an increase of 7.4%. It's a healthy growth rate, but the company's online business is dwarfed by rivals Google and Yahoo.

That's one reason why some analysts view as credible reports that Microsoft is eyeing a $500 million stake in the social networking site Facebook. It needs to find a way to quickly close the gap between itself and the Web 2.0 competition before it's left behind. "Social networking sites attract the majority of Internet users," said Lehman Brothers analyst Vijay Jayant, in a report issued Tuesday.

Jayant, however, notes that if Microsoft is bidding for a 5% stake in Facebook, as reported, it's backing the number two horse in a two-horse race. He notes that rival social networking site MySpace, in part owned by Rupert Murdoch's News Corp., "has more than twice the monthly unique visitors" than Facebook, even though Facebook is growing faster.

According to market watcher Comscore, MySpace averages about 68 million unique visitors per month, compared to about 34 million for Facebook. By contrast, Microsoft's attempt at a social network site, Windows Live Spaces, averages only 8.4 million unique visitors per month.

Microsoft already offers a number of developer tools and services that make it easier for users to enhance their Facebook pages. But owning a stake in the site would dramatically increase Microsoft's presence and influence in the social networking market. It would also provide a high-traffic outlet for the back end tools -- like Atlas -- that Microsoft acquired from aQuantive. Compared to MySpace, Facebook has taken "a sharper focus on building out its technology and providing a more open online platform for users and Web developers," notes Lehman's Jayant.

Despite the advice, Microsoft isn't saying whether it thinks that's all worth the reported $500 million for Facebook.