Knowledge management is considered an important factor for competitive advantage and sustainability for firms. As many knowledge management systems failed to achieve the desired results, enterprise social media (ESM) has received considerable attention as an alternative solution for knowledge sharing within a firm. This paper attempts to investigate the influence of various aspects of ESM on knowledge sharing. While previous literature mainly focused on structural aspects of ESM, this study focuses on social psychological aspects, such as social connectedness, social awareness, and social presence, along with reputational aspects (such as self-presentation). Further, in order to clarify knowledge sharing behavior, this study classifies knowledge sharing behavior into two categories, knowledge contribution and knowledge acquisition. The data used in this study was collected from 179 individuals who have experience in their own ESM. The results show that both social connectedness and self-presentation positively influence the two types of knowledge sharing behavior, i.e., acquisition and contribution. Meanwhile, social awareness turns out to be a significant determinant of knowledge contribution only. Contrary to our expectations, however, social presence does not significantly influence knowledge sharing behavior.