Evidence suggests varied motives drives individuals to use dual-purposed systems such as social media. Current models of IT acceptance do not account for all these motives, and, while these models have been used in the context of dual-purposed systems, the focus was usually on the productivity side of use. Building on theories of motivational needs and person-environment fit, we conceptualize the construct of user-system fit - a construct that comprises four dimensions of fit – user-expression, needs-supplies, demands-abilities, and user-group fit – and which we hypothesize is positively associated with IT use. A model is developed and tested, and the results indicate that user-system fit explains 32.2% of usage.