This paper presents an original approach to designing social media that support informal learning in the digital workplace. It adapts design-based research to take into account the embeddedness of interactions within digitally mediated work-based contexts. The approach is demonstrated through the design, implementation, and evaluation of software tools supporting a particular type of informal learning called "knowledge maturing". The paper: introduces and presents the rationale for, and concept of, knowledge maturing; presents a new design methodology for developing social media that support informal learning and knowledge maturing; focuses on one prototype, for "people tagging for organisational development", that was produced by the methodology (and concisely describes two others); presents the formative evaluation of the highlighted prototype; and finally, discusses the implications and insights arising from this work.