More and more people regard social networking sites as a major media to communicate with others. Based on theory of reasoned action, this study aimed to understand how usage motivations and site' privacy policy affect users' attitudes toward these sites, and how users' attitude and subjective norm influence usage intention. This study devised qualitative interviews to investigate into subjects' personal experiences and feelings of these social networking sites, and quantitative online survey with 350 samples to verify the proposed research model. The results showed that entertainment and social motivations, as well as privacy policy have influence on users' attitudes and that attitudes and subjective norm significantly affect users' intention. The findings will help the practitioners of social networking sites in planning effective marketing strategies.