The sellers include eBay Inc. and private equity firms Silver Lake and Andreessen Horowitz.

About 170 million people log in to Skype’s services every month, though not all of them make calls. Skype users made 207 billion minutes of voice and video calls last year.

Most people use Skype’s free calling services, which has made it difficult for the service to make money since entrepreneurs Niklas Zennstrom and Janus Friis started the company in 2003. An average of about 8.8 million customers per month, or just over 1 percent of the user base, pay to use Skype services.

Skype lost $7 million on revenue of $860 million last year, according to papers that the company has filed since announcing its intentions last summer to launch an initial public offering of stock. The IPO was later put on hold. Skype’s long-term debt, net of cash, was $543,883 at the end of 2010.

The Skype takeover tops Microsoft’s biggest previous acquisition — a $6 billion purchase of the online ad service aQuantive in 2007.

Microsoft said Skype will become a new business division headed by Skype CEO Tony Bates, who will report directly to Ballmer.

Although it makes billions from its computer software, Microsoft has been accustomed to losing money on the Internet in a mostly futile attempt to catch up to Google Inc. in the lucrative online search market. Microsoft got so desperate that it made a $47.5 billion bid to buy Yahoo Inc. three years ago, but withdrew the offer after Yahoo balked. Yahoo is now worth about half of what Microsoft offered.

Microsoft would be Skype’s second large-company owner. EBay bought Skype for $2.6 billion in 2005, but its attempt to unite the phone service with its online shopping bazaar never worked out. It wound up selling a 70 percent stake in Skype to a group of investors led by private equity firms Silver Lake and Andreessen Horowitz for $2 billion 18 months ago.

Besides eBay, Silver Lake and Andreessen Horowitz, Skype’s other major shareholders are Joltid and Canada Pension Plan Investment Board.

When Steve Jobs debuted the iPad 15 months ago, critical appraisals were all over the map, from effusive to dismissive, but I don’t think even the most gaga fanboy predicted that in little more than year the tablet would have meaningfully reshaped the entire personal computing industry.

But the symbolism here is even more powerful. In 1991, Apple was still pumping out popular products — that year the company introduced its first serious laptop, the PowerBook 100, along with its high-end Quadra and the iconic-looking Mac Classic II.

Then, in April 1992, Microsoft released Windows 3.1 and brought the mouse and multitasking to the PC masses. And that was that. Apple’s attempt to sue Microsoft for coopting the “look and feel” of the Macintosh in earlier iterations of the Windows operating system failed miserably, and for most of the 1990s, the company was an also-ran. Die-hard Apple lovers still claimed aesthetic superiority over the commodified Windows-Intel nexus, but they were like yapping Chihuahuas — indefatigable and noisy but hardly dangerous. Microsoft proceeded to throw its weight across the entire industry, crushing its competitors and even shrugging off the best antitrust efforts of Bill Clinton’s Department of Justice.

And yet now the iPad and the iPhone rule supreme — where litigation failed, a superior design philosophy has triumphed, at least for now. It’s one of the most extraordinary stories in the history of personal computing.

The move marks a major strategy shift for Nokia, which has previously equipped devices with its own software. Analysts said the deal was a bigger win for Microsoft than Nokia, whose CEO Stephen Elop in a leaked memo this week compared his company to a burning oil platform with “more than one explosion … fueling a blazing fire around us.”

Nokia said the partnership would “deliver an ecosystem with unrivaled global reach and scale.” However, it warned that the new strategy would also bring “significant uncertainties,” and said it expects profit margins to be hit by strong competition from rivals.

Nokia’s share price plunged 9 percent to euro7.43 ($10.11) in afternoon trading in Helsinki.

Elop, a Canadian national, joined Nokia from a senior executive position at Microsoft last year. The first non-Finn to lead Nokia, he is under intense pressure to reverse the company’s market share losses to North American and Asian competitors.

“Nokia is at a critical juncture, where significant change is necessary and inevitable in our journey forward,” Elop said. He added the company was aiming at “regaining our smart phone leadership, reinforcing our mobile device platform and realizing our investments in the future.”

Speaking later to analysts in London, he declined to say when Nokia would introduce a new device running on Windows Phone. But he said Nokia won’t bury its own Symbian operating system or the new Meego platform that it is currently developing.

The Symbian technology is being used in 200 million phones with 150 million more expected on the market, Elop said.

Android surpassed Symbian to become the world’s No. 1 smart phone software in the fourth quarter of last year, according to the Canalys research firm.

Microsoft CEO Steven Ballmer said the partnership would give the team “more innovation, greater global reach and scale.”

“We need to, and we will, collaborate closely on development … so we can really align and drive the future revolution of the mobile phone,” he said.

The key challenge will be to come up with devices of a quality level and hip factor that helps position Windows Phone as an attractive alternative to iPhone or Android.

Windows Phone 7, which was launched last year, still has a lot of catching up to do in terms of both the number of users and the number of “apps” available for the phones.

Nokia said its expertise in developing new software with Microsoft will be “on top of the platform in areas such as imaging, where Nokia is a market leader.” Its map services will be a core part of the new device as will Microsoft’s Bing search engine, Nokia said.

Neil Mawston of London-based Strategy Analytics said Microsoft was the big winner in the partnership, by teaming up with the biggest mobile hardware vendor in the world.

“In terms of expanding their distribution reach, this is a huge win for Microsoft,” he said.

For Nokia the deal leaves uncertainty about what will happen to its current Symbian operating platform. Mawston said he expects it to be phased out within two years and “completely, or at least mostly, replaced by Windows Phone.”

Although Nokia still is the mobile industry’s biggest handset maker, its market share has plummeted from a high of 41 percent in 2008 to 31 percent in the last quarter of 2010.

It has also lost its innovative edge in the fiercely competitive top-end sector and is virtually invisible — with a 3 percent share — in the world’s largest smart phone market, North America.

Apples’ iPhone has set the standard for today’s smart phones and Research In Motion Ltd.’s BlackBerrys have become the favorite of the corporate set. More recently, Google Inc.’s Android software has emerged as the choice for phone makers that want to challenge the iPhone.

“Today, developers, operators and consumers want compelling mobile products, which include not only the device, but the software, services, applications and customer support that make a great experience,” Elop said.

He warned of further layoffs and restructuring, saying Nokia must “improve the speed and nimbleness and agility of the organization … by taking significant steps in how we operate.” He gave no details.

The company said it will announce a new leadership team and organizational structure “with a clear focus on speed, results and accountability.”

Nokia, which claims 1.3 billion daily users of its devices, said it hopes the “broad, strategic partnership” with Microsoft will lead to capturing the next billion users to join the Internet in developing growth markets.

Jyrki Ali-Yrkko, from the Research Institute of the Finnish Economy, described Nokia’s cooperation with Microsoft as “surprising.”

“The strengths will be in Microsoft’s strong position in various corporate solutions and server solutions, but its weakness is that Microsoft perhaps doesn’t have a broad, user-oriented group of developers like those around Android or Apple,” Ali-Yrkko said.

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Online:

Nokia: http://www.nokia.com

Ozzie was the company’s Chief Software Architect, a position held previously by Bill Gates after he stepped down as CEO. It was an ideal fit: Ozzie’s technical talent and vision for what we could do with technology were extraordinary. At Microsoft he headed up an effort to move the company toward the era when software was more online than not, a sea change for a company that had for its entire existence been all about what amounted to packaged goods.

I’ve been an Ozzie fan for years. To journalists who covered his doings, he was patient in helping us understand what he was doing. Just what that was could be hard to grasp, given how far ahead of his time he proved to be on project after project at several companies including Groove and, before that, Lotus Notes.

For all his qualities, Ozzie didn’t push Microsoft fast enough toward the future, or else his pushing was resisted. Microsoft dallied way too long to get into the “cloud” where software becomes as much as service as a product you buy. The competition — Google, Amazon and others — is more entrenched now, and for all the formidable technical talent at Microsoft, the company hasn’t caught up in key areas. Keep in mind, however, that Microsoft’s bread and butter (and gold and diamonds) remains in the licensed-software market, where it’s still an absolutely huge and immensely profitable enterprise.

It’ll be fascinating to see what Ozzie does next. I find myself hoping he’ll try something in the social-entrepreneurship arena. Certainly he can live with a lower paycheck than most of us.

As for Microsoft, which keeps losing (or expelling) top executives, the questions grow more urgent. Ballmer has been a better CEO than his critics say, but if the board isn’t pushing him to line up a solid successor, and soon, the directors are falling down on the job.

But Sony and Nintendo still had to deal with that pesky speed bump of a disc. Any PS3 user knows the pain of having to eject a game and insert the Netflix-provided disc every time they wanted to watch a movie. In a world of efficiency and an increasingly lazy population, this was a major hurdle and shifted some prospective buyers to the Xbox.

 Now they’re on equal ground, and even though the PS3 is late to the disc-less game it will offer dramatically more than either Nintendo or Microsoft, including a renovated interface, faster start times, Dolby Digital 5.1-channel surround and 1080p hi-definition support. PC World calls it the best version yet. 

Now the world waits for Xbox’s response.