For starters, the firm gets well over 600 million users who make Skype the world’s largest phone company for international voice calls.

More importantly, Microsoft buys into a lot of potential.

Marry Skype’s software with the Xbox Kinect and an HD television set, and Microsoft can make a powerful argument for getting into millions of living rooms.

Think beyond teleconferencing for the whole family: there’s one-on-one training, home schooling, even patient care delivered remotely and in vision.

Smarten it up for the corporate world, and Microsoft can challenge the telepresence business of firms like Cisco and Polycom.

And Skype is multi-platform, reaching into the worlds of Apple and Linux.

Finally, Skype is mobile, and can be paired with Windows Phone 7.

The hitch: Microsoft boss Steve Ballmer will have to work hard to integrate Skype, to ensure the voice/video-over-the-internet company is not strangled by his firm’s notorious bureaucracy.

Microsoft chief executive Steve Ballmer said: “Skype is a phenomenal service that is loved by millions of people around the world.

“Together we will create the future of real-time communications so people can easily stay connected to family, friends, clients and colleagues anywhere in the world.”

Skype will now become a new division within Microsoft, and Skype chief executive Tony Bates will continue to lead the business, reporting directly to Mr Ballmer.

Analysts say Microsoft’s aim in buying Skype is to improve its video conferencing services.

Although the price tag of US$8.5b will not stretch the US giant, some experts have questioned whether it is paying too much for a company that has struggled to turn a profit.

Michael Clendenin, managing director of consulting firm RedTech Advisors, said: “If you consider [Skype] was just valued at about US$2.5b 18 months ago when a chunk was sold off, then US$8.5b seems generous.

“[It] means Microsoft has a high wall to climb to prove to investors that Skype is a necessary linchpin for the company’s online and mobile strategy.”

Skype was founded in 2003.

Calls to other Skype users are free, while the company charges for those made to both traditional landline phones and mobiles. (BBC)