What happens when you create the most downloaded file-sharing client only to sell it off a year later? Certainly not sit around and hope for the best. Niklas Zennstrom and Janus Friis, creators of the FastTrack network, would go on to develop the wildly popular P2P network better known as Skype.

Skype isn’t your traditional P2P network by any means - although its foundation is largely based on the FastTrack network. Skype is a VoIP P2P network that allows communications in real time. While supernodes typically index available files, with Skype they index and connect available users on the network. From there it’s simply a matter of populating a contact list with friends and talk, and like P2P, for free.

Since its introduction, populating a contact list has grown steadily easier as Skype is quickly becoming mainstream. With over 4.88 million users on the network at the time of this writing, it has surpassed FastTrack’s peak population of 4.5 million users.

Unlike the legal questions and wrangling that continue to plague FastTrack and Kazaa, Skype has managed to find better financial backing and success. While Sharman’s purchase of the intellectual property right to the Kazaa software undoubtedly brought a small fortune to Zennstrom and Friss, it pales in comparison to eBay’s 2.6 billion dollar purchase last year.

Although Zennstrom and Friis have found the proverbial pot of gold, the road to success has not been completely smooth. Because Skype is built on the FastTrack network, security concerns remain paramount. Skype may claim Skype is secure with its purportedly strong encryption, but like FastTrack the network is closed and proprietary. This leaves little room to verify the authenticity of its security which has left many skeptical. In addition, StreamCast has recently filed a civil complaint under the RICO act against Zennstrom and Friis.

Yet like FastTrack, these security concerns and possible vulnerabilities have not prevented wide-scale support and adoption. Skype has quickly risen through the Internet ranks to become one of the most popular software applications. The company reported their 100 millionth registered user yesterday, an achievement accomplished in only 2.5 years time. Kazaa achieved a similar milestone, being the only application downloaded over 300 million times.