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Today, instant messaging has been becoming a major communication technology in business environment. One important causal factor related to the use of instant messaging is critical mass. Specifically, the use of instant messaging requires collective efforts and interdependence among two or more people, and thus is conceptualized as intentional social action in the current study. Drawing on prior studies of social influence and critical mass, we propose a research model and empirically evaluate it using survey data collected from 227 respondents. The results indicate that perceived critical mass impacts usage we-intention both directly and indirectly through attitude and social influence factors. We believe the findings of this study will provide important implications to both researchers and practitioners.