(Bloomberg News) David Einhorn's call to replace Microsoft Corp. Chief Executive Officer Steve Ballmer adds to the drumbeat for change at the top of technology bellwethers.

Boards at increasingly large hardware and software makers are replacing CEOs to help their companies repel threats from upstarts such as Facebook Inc. and Apple Inc. The aim is to keep from getting left behind in emerging technologies including social networking, mobile computing and the delivery of software over the Internet, via the so-called cloud.

Hewlett-Packard Co., Google Inc. and Advanced Micro Devices Inc. lead technology companies with a combined $265 billion in market value on the Standard & Poor's 500 Index that have changed CEOs since August. That's up from companies worth $75 billion a year earlier. Privately held Twitter Inc. replaced its CEO in October, a month after Finland's Nokia Oyj did the same.

Investors may clamor for new leadership at companies including Cisco Systems Inc. and Research In Motion Ltd., said Bill Coleman, a partner at venture capital firm Alsop Louie Partners in San Francisco.

"The companies that haven't moved are under a lot of pressure, and one of the first places you look to change things is at the top," said Coleman, the former CEO of BEA Systems Inc., which was acquired by Oracle Corp. "There are lots of companies like Nokia and RIM and AMD and Cisco that have fallen behind and lost at least some of their competitive edge."

Ballmer is being criticized as Redmond, Washington-based Microsoft loses market share to Apple and Google in mobile phones and Apple's iPad takes sales from personal computers running Microsoft's Windows.

Ballmer Dinged

The board docked Ballmer some of his potential bonus last year for falling short in mobile and new forms of computers.

Einhorn, president of hedge fund Greenlight Capital Inc., said Ballmer has failed to seize on Microsoft's opportunities and overspent on efforts to remedy shortcomings.

"He's allowed competitors to beat Microsoft in huge areas, including search, mobile-communications software, tablet computing and social networking," Einhorn said at a conference this week. "Even worse, his response to these failures has been to pour tremendous resources into efforts to develop his way out of these holes."