As mobile instant messaging (IM) has become the fastest-growing social media (e.g. WeChat, WhatsApp), this study investigates the use of it as a virtual collaboration tool in multi-organization projects. The understanding of the relationships among tool, task and team in technology-facilitated collaboration leads to the integration of technology acceptance model, task-technology fit model and network externality framework into a research model. Tool usability, task fit and team connectivity are hypothesized to have direct and indirect effects on virtual collaboration effectiveness. The results based on survey observations collected from multi-organization project teams using WeChat in China support most of the hypothesized relationships. The theoretical and practical implications of findings are discussed.