(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."

During Ballmer's 11-year reign as CEO, shares have declined more than 50 percent, even though sales have more than tripled and profit has risen 141 percent.

While investors' patience with Ballmer is wearing thin, Microsoft's board may not quickly agitate for change, said Pat Becker Jr., principal of Portland, Oregon-based Becker Capital Management, which holds Microsoft.

'Losing the Tech World'

"He doesn't have the investor base and my fear is he's losing his customer base and losing the tech world in general," said Becker, whose firm has about $2.5 billion in assets. "The investor base would welcome new leadership, but whether the board wants him to go is a different thing."

Ballmer's supporters could point to the company's sales and profit growth on his watch, said Michael Cusumano, a professor at the Massachusetts Institute of Technology's Sloan School of Management in Cambridge.

"Financially Microsoft has really done quite well," Cusumano said. "They are still printing money essentially."

Frank Shaw, a spokesman for Microsoft, declined to comment.

Other boards have shown less resistance to change. Hewlett- Packard directors pushed out former CEO Mark Hurd in August after an investigation found he violated the company's code of business ethics by concealing a personal relationship with a female contractor.

Hewlett-Packard, AMD, Google

Hurd's successor, Leo Apotheker, said this month that he inherited a company ill-equipped to win business from companies that want to switch to cloud computing. AMD board members ousted CEO Dirk Meyer in January amid frustration with the company's lack of progress in chips for tablets.

Google turned to one of its co-founders, Larry Page, 38, to succeed Eric Schmidt, 56, who ran the company for a decade. The move was aimed at helping Google bolster its defenses against Facebook and recapture the entrepreneurial ethos that fostered the creation of the most-used search engine.

Some companies aim for executives who, by dint of age or strategic vision, convey the sense they can cater to younger, technology-savvy consumers, said Paul Saffo, managing director at investment adviser Discern Analytics.

Under CEO Steve Jobs, Apple has gained share in the digital-music and mobile-phone industries with products that demonstrate his appreciation of customers' preferences, says Saffo, whose firm is based in San Francisco.

Knowing Customers' Needs

"There's that notion that a younger perspective really matters," he said. "It's the difference between understanding new technologies intellectually versus intuitively. It's not necessarily an age thing. Look at Steve Jobs -- he's old enough to join AARP -- but he has an intuitive understanding."

Cisco CEO John Chambers, who scrapped a longstanding target for annual sales increases of as much as 17 percent earlier this month after five straight quarters of disappointing earnings reports, "could be the next target," Cusumano said.

Chambers is taking steps to revive growth by eliminating jobs, exiting lower-margin consumer businesses and dismantling a management structure that slowed decision making. Still, the shares have yet to reverse a slide that has left them 29 percent lower in the past 12 months, compared with a 24 percent gain in the Standard & Poor's 500 Index.

"People worry Cisco has lost their edge and John Chambers isn't what he used to be," said Dan Morgan, a fund manager at Synovus Securities Inc., which oversees $7.5 billion and cut its Cisco holdings this year, according to Bloomberg data.

Karen Tillman, a spokeswoman for San Jose, California-based Cisco, declined to comment.

CEO 'Term Limits'

Some CEOs have spent too long in the job to be able to undertake the continual reinvention needed to keep pace, said Ed Zander, who served as CEO at the company then known as Motorola Inc. and chief operating officer at Sun Microsystems.

"I've always thought there ought to be some term limits on CEOs," said Zander, who stepped down in 2008 as Motorola's CEO. "There's more questions and more focus on the management ranks, and do they have the wherewithal to keep reinventing. Some CEOs have been at it a while and it's hard."

Analysts from at least eight securities firms have cut their ratings on RIM after the Waterloo, Ontario-based company reduced profit forecasts late last month, adding to evidence that the maker of the BlackBerry is struggling to compete against Apple and Google in the smartphone market.

Lazaridis' 'Technological Brains'

Sameet Kanade, an analyst at Northern Securities Inc. in Toronto, has suggested the company should scrap its dual-CEO structure, elevating co-CEO Mike Lazaridis over Jim Balsillie.

"When the momentum was in their favor, not a lot of attention was paid to the disconnect of a co-CEO structure, but with it, accountability is divided, or is not focused on one person," Kanade said. "Right now, they're facing technological challenges and Lazaridis, the technological brains, should be running the whole show."

RIM's share of global smartphone sales fell to 13 percent in the first quarter, from 20 percent a year earlier, Gartner Inc. said. Share for Google's Android more than tripled to 36 percent from 9.6 percent and Apple's iOS rose to 17 percent from 15 percent. RIM stock has plummeted 27 percent in the past year.

Marisa Conway and Tenille Kennedy, spokeswomen for RIM, didn't respond to requests for comment.

"There's market impatience with anyone who is not No. 1 in their area," Saffo said. "With all the technology shifts, the degree of uncertainty has increased dramatically. Executives have never had more pressure on them."