As the number of Chinese internet users grows very fast, the online industry of China continuously develops and online instant messenger(IM) became the most used communication tool in China. Until the first half of 2010 the number of internet users in China reached 420 million, and total active IM user accounts surpassed 636 million by the third quarter of 2010. Tencent`s QQ has been at the center of such an explosive increase of IM users in China. So this study tried to take a general approach to IM use pattern of Chinese young people as well as a specific approach to QQ IM users behavior through the field survey. In addition this study classified QQ IM users into four groups whose entry motive is related with network effect by applying their opinions for the satisfaction and advantage of QQ. According to the survey result online network including IM already became an inevitable communication method at least for Chinese young people. And the competitive advantage of QQ IM based on network externality and portal site lock-in effect will be kept as long as the number of Chinese internet users grows. However to go abroad strategy of Tencent s QQ will be not easy because of the liabilities of foreignness such as language barriers or cultural difference. In future as wireless internet and smart phone are gradually popular in China the online IM will make a big contribution to the arrival of Web 2.0 period . Furthermore the portal site competition among Chinese local firms will be changed by online platform competition.