Stiff competition in the mobile instant messaging (MIM) market underscores the importance of continuance use of MIM to ensure sustainable growth of service providers. This paper investigates the impact of perceived usability, perceived security, perceived service quality, and confirmation on users' continuance intention to use MIM. A research model based on the expectation-confirmation model was built and empirically tested using data from 334 MIM users in South Korea. The results showed that perceived service quality and perceived usability significantly affect user satisfaction and continuance intention to use MIM. Perceived service quality also influences confirmation, which in turns affects perceived usability. The effect of perceived security on user satisfaction, however, is not significant. The findings provide insights to service providers when strategizing to increase customers' continuance intention to use MIM.