As smartphones become more and more popular, uses of social media on mobile devices have grown explosively. Previous studies have investigated usage behaviors of social networks in various ways, but few of them emphasized the activities on smartphones and social applications. In this study, we develop a research framework based on the network externalities and cognitive absorption to explain users’ actual utilization and satisfaction of a popular social mobile messaging app in Asia: LINE. Two different contexts - information sharing and game playing were examined in this research. We tested our research model using 331 data collected via online survey and we found that (1) network externalities (the number of peers) had a positive effect on an individual’s actual usage; (2) cognitive absorption positively affected an individual’s actual usage of and satisfaction toward LINE; (3) LINE users’ actual utilization had a significant impact on their satisfaction; (4) LINE users’ intrinsic motivation (measured by cognitive absorption) is more important than extrinsic motivation (measured by network externalities); and (5) tasks with higher hedonic motives such as game playing may be a stronger trigger of LINE users’ actual utilization.