In its second full week on the market, Microsoft's new Bing search engine continued to steal usage share away from Yahoo! and Google. But Microsoft CEO Steve Ballmer cautioned against any unreasonable optimism, noting that the service has a long road ahead of it.

"We have had some very good initial response," Ballmer said this week. "I don't want to over-set expectations. We are going to have to be tenacious and keep up the pace of innovation over a long period of time."

"We may be successful, we may not, but we can't be successful without being committed to changing things, changing the approach, changing the business model, and you can't give up in six months, or a year or two years," he added.

Bing grabbed over 12 percent of the US market for Internet searches this week, up from 11.3 percent the week Bing launched and 9.1 for Live Search the week before. Prior to that, Microsoft's Internet search service had steadily lost usage share; the service had 8.2 percent share in April.

Of course, Bing usage still pales in comparison to market-leader Google. Google's more general search service continues to dominate. While exact figures for last week are not yet available, Google accounts for about 65 percent of US Internet searches, while number-two Yahoo! averages about 20 percent.

Microsoft positions Bing as a "decision engine" instead of a general search engine, though it does offer standard Internet search capabilities. Where Bing really excels is in vertical, complex searches on topics such as travel plans, health care, and shopping.

Related Reading:

• Bing Beta Review Part 1: The Making of a Decision Engine
• Bing Off to a Fast Start
• You've Been Binged: Google Goes on the Defensive
• Badda Bing! Microsoft Reveals Next-Gen Live Search