Over the last decade, a plethora of Enterprise Social Software (ESS) has emerged in various shapes, yet difficult to compare what they enable or constrain their users to do. Neither the prior frameworks nor the ambiguous concepts shed light on the fine-granular similarities and differences among them. In particular, organizations can consciously design and adjust their ESS artifacts. Hence, it is relevant to assess the possibilities for goal-oriented action they offer and spot the differences among them. Following a structured method, we identify eight distinct dimensions with subordinate characteristics that enable the classification of functional affordances of ESS. This paper presents the resulting taxonomy that has been built and evaluated over six iterations. We contribute to practice by supporting practitioners to assess ESS, inspire the innovation of existing ESS and the development of future ESS. Furthermore, we build a foundation for future research to systematically develop and investigate ESS.