Many researchers have examined the technology acceptance model (TAM) that Davis (1986) created to predict the voluntary use of information systems. However, TAM\u27s primary focus is on how ease of use and usefulness influence acceptance, without looking at the effects of network externalities. In this study, we examined adoption behavior involving Internet-based instant messaging services (IMS). A questionnaire was used to collect data on perceived usefulness, perceived ease of use, perceived number of users for external network utility, and technology utility. The results indicate an acceptable goodness-of-fit statistic for our proposed TAM model, which combines the original TAM concept and network externality theory. The results also support the importance of network externalities in considering IT acceptance