With the rapid growth of population of Internet, social network site has become a communicable platform that people share some information themselves or connecting whit friends, such as MySpace, Facebook, Twitter. This study uses TRA and TAM as basic model, and combined with the Social Identity Theory to explore what factors affect internet users to stick on the social network site. The study adopts the Facebook as experiment platform, and collects 318 Taiwanese samples from internet. After the analysis of structural equation model, main results are the followings: (1)Users’ social identity, perceived usefulness, perceived entertainment, attitude toward a social network site will significantly and positively affect their stickiness on a social network site; (2)Users’ perceived usefulness, perceived ease of use, perceived entertainment will significantly and positively affect their attitude toward a social network site; (3)Users’ social identity will affect their perceived usefulness and perceived entertainment toward a social network site.