Most of the existing literature on empirical studies of Online Social Networks (OSNs) have focused on characterizing and modeling the structure of their inferred friendship graphs. However, the friendship graph of an OSN does not demonstrate what fraction of its users actively interact with other users, how these users interact, and how these active users and their interactions evolve over time. In this paper, we characterize indirect fan-owner interactions through photos among users in a large photo-sharing OSN, namely Flickr. Our results show that a very small fraction of users in the main component of the friendship graph is responsible for the vast majority of fan-owner interactions; moreover, these interactions involve only a small fraction of photos in Flickr. We also characterize some of the temporal properties of fan arrival. For example, we show that there is no strong correlation between age and popularity of a photo and that most photos gain a majority of their fans during ...