Along with the growth in artifact sharing in online communities such as Flickr, YouTube, and Facebook comes the demand for adding descriptive meta-information, or tags. Tags help individuals to organize and communicate the content and context of their work for themselves and for others. This longitudinal study draws on research in social psychology, network theory and online communities to explain tagging over time. Our findings suggest that tagging increases as a contributor receives attention from others in the community. Further, we find that the more a user's network neighbors are connected to each other directly, the less the focal user will tend to tag his photos. However, density interacts with attention such that those who are surrounded by a dense ego network respond more to attention than others whose ego networks are sparsely interconnected. Unexpectedly, we find no direct correlation between tagging and the individual motivations of enjoyment and commitment. While commitment is not directly associated with tagging, there is an interaction effect such that the effect of commitment on tagging is positive for users with low-density ego networks and negative when a user is surrounded by a high-density network. Directions for future research as well as implications for theory and practice are discussed.