The more people who adopt a particular technology, the more valuable the technology is for the other users. For example, the more people who have a telephone, the more valuable telephones are to everyone else. Another similar example is email: the more people who have access to email, the more valuable it is to everyone else.The more people register onto a social network website the more people useful the website is to its registration. Apple could be another example. The iPhone 4 has proven to be a popular item in China since its launch last weekend.

China Unicom sells 100,000 iPhone 4s in first four days. IDG News Service reports that China Unicom has sold nearlyjavascript:void(0) 100,000 iPhone 4 units over the last four days. On Tuesday, company Vice President Li Gang told reporters that the carrier had essentially sold out of its first shipment.
website: http://www.macworld.com/article/154419/2010/09/iphone4_china_100k.html