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The D&M IS Success Model has been widely applied to evaluate the information system now. Since the connections and communications among individuals tend to be closer, this study introduces network externalities to the existing model In addition, although previous studies view the information quality, system quality and service quality as independent variables without interactions, this research investigates the inter-relationships among them. Since there are so many applications with same functions in the competitive market, in this study we also try to understand competing application usage by integrating IS success model to see how individuals make a decision to select one from two choices, and analyze the theory in the relative evaluation by applying comparison by difference (e.g., “user’s satisfaction towards A” minus “user’s satisfaction towards B”). The result shows that network externalities have great significance on three dimensions of quality, and its effect on service quality actually exerts by the mediating effects of information quality and system quality. The service quality was also proved to be affected positively by information quality and system quality. Thus, the current study not only contributes to the current theory by extending the model, but also provides some practical implications.