It is important for all types of organisations including non-profit organisations (NPOs) to manage knowledge for effective and efficient utilisation of resources. Technology is considered as one of the key enablers of knowledge management (KM) practices but it can be costly to develop and implement in an organisation. With the advent of social media, NPOs such as public libraries have the opportunity to harness the power of technology for KM purposes as it is considered a low cost medium. A study was conducted, using an exploratory qualitative interview technique, in two contrasting public libraries: one is a large urban public library, and the other is a small rural public library. The data were analysed using a grounded theory approach informed by a social constructionist theoretical framework. This paper presents comparative findings from these case examples on their understanding of KM as a concept and their use of social media in management of knowledge. Results show that social media are valuable KM tools in public libraries, not only when directed externally for the purpose of promotion, but also to foster engagement with the public and collaborative work within the organisation.