Charles Di Bona, a software research analyst at Sanford C. Bernstein, said Ballmer's comments did not give enough details about how that additional investment will be spent and how the company arrived at that decision.
"It's spending $500 million dollars and then it says we'll tell you later how we'll spend it," said Di Bona, who has an "outperform" rating on Microsoft.
"The market's concern is not about how it is running its core business. It's about decisions about larger chunks of money that people can't track."
Ballmer said Microsoft is willing to endure online division operating losses that amount to between 5 percent to 10 percent of the company's total operating income, which reached $22.5 billion in fiscal year 2008, until the search and advertising business reaches "scale."
He did not specify on how long this period of losses would last, but said the risk was worth the potential return.
Microsoft's online division has posted eight straight quarters of losses. It lost $1.23 billion in the past fiscal year, twice as much as it had lost in fiscal 2007 and about 5.5 percent of Microsoft's total operating income.
Ballmer said its online businesses could eventually account for most of the economic value created by the world's largest software maker.
The Microsoft CEO was left to describe Internet strategy after Microsoft announced one day before the analyst meeting that the head of that business, Kevin Johnson, was leaving. He will become chief executive of Juniper Networks.
"We thought it was important whoever was going to get up and talk about the big investment online was going to be here in three weeks and so you're stuck with me on this topic today," Ballmer said.
Big Plans Without Yahoo
Seeking to show momentum in its existing Internet business, Microsoft announced that it had expanded its existing pact with Facebook, the world's largest social networking site, to provide Web search and search advertising in addition to its existing deal to run graphical display ads on Facebook pages.
Satya Nadella, Microsoft's senior vice president in its search and advertising group, said the expanded Facebook deal would be implemented in the next few months and "carry both our Web results, as well as our page search advertising."