'I would suspect that could have achieved more if it had been an independent company,' said Danny Rimer, a partner at Index Ventures, which invested in Skype in 2004. ( Read more: Microsoft changes password system to deter Skype hacks) This could provide lessons for other start-ups that became high-profile acquisitions. Some observers say the lack of recent innovation could be traced to Skype becoming just one part of a much larger company. While messaging apps have integrated with other services, Skype's revenues continue to come entirely from calls.

Instant messaging was always seen as a secondary feature,' says Henn Ruukel, an engineer who has left Skype to set up a messaging service, Fleep. 'Good video calling was the number one priority. Instead a new crop of chat app start-ups, such as Viber and WhatsApp, have become the mobile messaging tool of choice, especially in Asia. Skype never dominated instant messaging on mobile, despite promising as early as 2007 that it would do 'much more than voice'. In 2005, a year before Twitter emerged, Skype started allowing users to enter a mood message alongside their name, but it did not expand the feature. ( Read more: Microsoft 'a modest growth business': Pro) Yet that success could have been much broader. It has had a particular impact on international calls, traffic growth for which has slowed a lot since 2008. Skype had 'more than twice the volume growth achieved by all the phone companies in the world combined' last year, according to Telegeography, a research organisation.

That soon became a double entendre when call quality turned out to be volatile, yet within a month it had been downloaded 500,000 times. Its slogan was 'Free internet telephone that just works'.

The software launched on Augfrom a Soviet-era university block in Tallinn, Estonia. Skype aimed to put traditional telephone companies out of business by making all calls free, leading one analyst to liken it to 'a giant meteor hurtling on a collision course toward earth'. The last meaningful thing to be launched by Skype was video calling in 2005.' 'In some ways Skype is a victim of its own success,' said Taavet Hinrikus, the company's first employee.

( Read more: Microsoft's Skype deal needs changes: Cisco CEO)

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