Flickr, the large-scale online photo sharing website, is often viewed as one of the ‘classic’ examples of Web2.0 applications through which researchers are able to observe the social behavior of online communities. One of the main features of Flickr is groups. These provide a means to organize, share and discuss photos of potential interest to group members. This paper explores the scale of group creation on Flickr and proposes a new set of metrics for characterizing groups on Flickr looking at aspects of membership, communication activity, and communication structure. Data collected from a sample of 1,000 groups was used to confirm the metrics and provide new insights into group formation in Flickr, such as the nature of larger and smaller groups. The contributions of the article are as follows: a set of metrics for characterizing online groups that extend existing schemes; an approach for sampling Flickr to estimate the number of groups; new insights into Flickr groups based on results from analyzing 1,000 randomly selected groups; and reflections on our experiences with using publicly accessible, automatically collected data to characterize the types of groups on Flickr.