This doctoral dissertation investigates how unions may revitalize by moving from existing ‘solid’ organizational models to new ‘liquid’ forms of unionism. We define the solid model of unionism as bureaucratic, low-intensive in digital technologies, predictable and highly formalized and embedded in long-term horizons. By contrast, the liquid unionism is defined as networked-based, highly digitalized, improvisational and in-the-making and much more ephemeral. More precisely, this dissertation studies such a transformation process from solid to liquid by considering unions as both social movements and democratic organizations. To answer this question, we explore three cases of union renewal attempts in North America. The three chapters of this dissertation rely on mixed methodologies, combining interviews with both qualitative and quantitative data harvested from social media. Our empirical investigations comprise one case of union liquefication failure and two cases of successfully fluidified unions. Chapter one discusses the case of a union which failed to renew its democratic model by introducing social media. This failure is centrally attributable to the invasion of the union Facebook group by ‘trolls’, which disrupted the union’s existing democratic process. The concept of a monstrification process is then proposed to make sense of this brutal and noxious confrontation between two democratic approaches, namely the union's existing solid model and the new liquid model realized through social media. Chapter two discusses an original type of collective action adopted by the US Fight for 15 labour movement. We introduce the metaphor of ‘flash mob unionism’ to describe this renewed and semi-liquid form of collective action, in reference to the contemporaneously emerging genre of artistic performance. Chapter three compares the success factors of two large-scale labour movements: Fight for 15 and Our Walmart. We suggest that these two movements succeeded because they both moved from a solid ‘organizing’ paradigm to a more liquid ‘networking’ one. This dissertation contributes to the literature on unionism renewal in three ways. First, it suggests three metaphors (the monster, the flashmob, the network) to make sense of the process of union transformation from a solid to a liquid model. Second, it suggests different factors which may facilitate or inhibit this transformation process. Third, it evidences the potential for enrichment of industrial relations and organization studies inherent in methodological approaches based on social media data.