Argued to be a crucial process in knowledge management, tacit knowledge externalization refers to employees activities for converting their unarticulated, personal tacit knowledge into explicit, well-articulated knowledge. For innovative, knowledge-based organizations, providing employees with rich social interaction opportunities necessary to externalize their tacit knowledge for future use by others is essential. In response, identifying rapid, effective means to externalize employees tacit knowledge has become a major objective for organizations. In such a vein, organization have started introducing corporate social media platforms. By bundling wiki, blog and social networking technologies and their various functionalities, organizations aim at supporting employees social interaction necessary for tacit knowledge externalization. However, the value of such platforms for employees tacit knowledge externalization remains unclear so far. In response, this cumulative dissertation consisting of four research articles first identifies tacit knowledge externalization mechanisms and activities enabled by corporate social media platforms and develops a categorization and performs a holistic operationalization of such mechanisms and activities. From that operationalization, an instrument to assess employees tacit knowledge mechanisms and activities is developed and validated in different corporate social media contexts. With that instrument, the dissertation altogether examines knowledge use as an outcome and high-performance work practices as antecedents of tacit knowledge externalization on corporate social media platforms.