Informal Learning in the Workplace (ILW) is ensured by the everyday work activities in which workers are engaged. It accounts for over 75% of learning in the workplace. Enterprise Social Media (ESM) are increasingly used to promote informal learning environments. In this paper, we discuss the principles and features of social media, and present to what extent they promote informal learning in the workplace. We propose also a user-centred design methodology to redesign a traditional knowledge-sharing tool with social features. We then present an implementation, in a real context, of the methodology. It shows that ESM are appropriate to promote ILW. Three adaptations must nevertheless be carried out: (1) Base the design on a precise and relatively exhaustive informational corpus and contextualize the access in the form of community of practice structured according to collaborative spaces; (2) Add indicators of judgment on the operational quality of information and the informational capital built, and (3) Define forms of moderation and control consistent with the hierarchical structures of the company. Our analysis also shows that an incremental and iterative approach of user-centered design had to be implemented to define how to adapt the design and to accompany change.