SIOC --- Semantically-Interlinked Online Communities --- provides the Semantic Web with a vocabulary for representing activities and contributions of online communities. However, it focuses on the state of online communities at a given time, while a number of Web application put a strong emphasis on the dynamics of their components and users, including microblogging, status and geolocation notification, etc. and as also testified by recent efforts on modelling activity streams. This work proposes a new module for the SIOC vocabulary designed to represent the dynamics of actions within online communities. Hence, we provide an action-centric view of online communities, while previous work focused on a document-centric or user-centric one. Furthermore, we align our work with related vocabularies and Web technologies --- both in use and emerging --- inside and outside the field of Semantic Web technologies.