We think a lot about network effects: the positive externalities caused by greater connectivity. A telephone network with 100 subscribers offers 99 calling opportunities whereas one with 10 subscribers offers only 9. That is why regulators had to fight so hard to ensure seamless interconnection that would give the subscribers on each network 109 calling opportunities and compel the operators to compete on some other aspect of service. Here below is a discussion of network effects in Face Book, that is among other things, causing us to place advertisements on it. For an individual member, the most powerful network effects may be indirect ones that come from the huge number of unknown other people in the Facebook world.
Here is what Telegeography has to say on the subject: Computer-based Voice over IP (VoIP) is nothing new, but Skype is the first such service to break into the mainstream, attracting millions of users worldwide. Skype had 1 million simultaneous users within six months of the release of its first version for Windows in July 2004. By the end of the third quarter of 2006, Skype had 136 million registered users, and the number of users online now regularly exceeds 8 million. These users generated about 6.6 billion minutes of traffic in the third quarter of 2006, and are on track to make over 27 billion minutes of PC-to-PC calls this year.