The social media are often perceived as tools that support openness and flexible participation among individuals and communities. This might explain the reason why social media have become indispensible for many daily practices in organizations. But how do these organizations appropriate and use these media relative to their formal structures and characteristics is a question in focus for the current paper. Drawing on classical concepts of organizational bureaucracy from Weber (1978) and Eisenstadt (1959), we present a qualitative analysis of empirical data obtained from two large organizations that use a wiki as a collaborative knowledge platform. The results show how the tendency to organize the use of the wiki through introducing structure might create barriers for open and democratic collaboration and knowledge sharing at the workplace. They also show that while a freer approach to using wikis might allow for self-organizing, there is still a possibility for enacting social structures that limit openness and flexibility. As such, the paper contributes novel insights into how social media might be used in bureaucracies and soft bureaucracies.