Due to the popularity and intensive usage of Online Social Networks, more and more companies try to get in touch with existing as well as potential customers and to market their brands and products within these networks. Especially Facebook provides an extensive user base and the opportunity to implement fan pages, which allow companies to leverage a broad range of technical features and media types to interact with users. However, the success of company fan pages varies and is by far not guaranteed. Therefore, it is necessary to measure whether a company's efforts to stimulate user activity on fan pages are successful or not. Hence, we deduce and hypothesize factors influencing the number of active users on fan pages and take different technical features and media types companies can use into account. To test our hypotheses empirically, we draw on publicly and non-publicly available data of a global insurance company's fan page within Facebook. The results suggest that based on the number of current fans companies can particularly stimulate user activity and engagement by using company wallposts with its different media types as well as by commenting on already existing wallposts. Finally, both theoretical and practical implications are discussed.