We propose a decomposed expectation confirmation model of IT continuance by 1) decomposing user expectation and confirmation into two dimensions of usefulness and usability, 2) conceptualizing the role of usability in IT continuance research, and 3) exploring changes in user perceptions of usability and usefulness over time and their impacts on user satisfaction and continuance intention. We tested the proposed research model using longitudinal data that we collected in two points in time six months apart from 125 users of the LinkedIn professional social networking site (pSNS). The results show that 1) perceived usability, usability confirmation, perceived usefulness, and usefulness confirmation determined user satisfaction with pSNS and 2) perceived usability along with satisfaction predicted continuance intention, whereas perceived usefulness had no effect. This study contributes IT continuance research by 1) proposing and empirically validating a decomposed model of IT continuance and 2) by bringing in usability as a core construct of interest for IT continuance research.