In this paper, we explore how a social bookmarking service is used to support knowledge sharing in a large enterprise. While there has been considerable interest in social bookmarking and collaborative tagging systems in recent years, very little is known about their actual usage. In this paper, we present the results of a field study of a social bookmarking service that has been deployed in a large enterprise. We present new quantitative data on how a corporate social bookmarking system was used, through in-depth analysis of the bookmark and tag data. We specifically explore the patterns of information sharing and social interaction for a twelve month period of time across three groups within a large enterprise. We discuss how the results of our investigation can used to better support communities of practice with social bookmarking services.