Thanks to the advancements in Social Media, Knowledge Management is evolving to ways of implementation based on active participation of employees and free knowledge sharing. This shift is deemed to allow overcoming some limits of the traditional approach, and especially the ineffective management of tacit knowledge. The KM literature has often analysed the benefits and advantages of Social Media, but less attention has been devoted to their limitations and problems. The paper contributes to this topic by examining an unintended consequence of the use of Social Media: the “cognitive overload” effect. The paper analyses the experience of community managers (that is, the managers of the internal Communities of Practice) at a big consulting company that uses Social Media to support knowledge generation and transfer within its Global Communities. The study highlights the risk of cognitive overload and its implications. The research makes use of the case study method and focuses on the analysis of the particular Enterprise Social Media platform used by this company. A first contribution and original aspect of the paper is that it focuses on the problems and drawbacks of Social Media in KM that have not been properly addressed in the literature. Also, it provides an analysis of a real-life situation. It examines the notion of cognitive overload and related risks, which extends the antecedent notion of “information overload” and is particularly appropriate to explain potential drawbacks of Social Media in business. The analysis provides lessons for companies willing to use Social Media to support innovative approaches to KM, and can also give inspiration for future research in this field. Particularly, the study highlights the main problems that are associated to cognitive overload induced by Social Media, and suggests possible measures to mitigate them.