Why do people tag?

Introduction Tagging, or using keywords to add metadata to shared content, is gaining much popularity in recent years. Tags are used to annotate various types of content, including images, videos, bookmarks, and blogs, through web-based systems such as Flickr, YouTube, del.icio.us, and Technorati, respectively. The popularity of tagging is attributed, at least in part, to the benefits users gain from effective sharing and from organization of very large amounts of information. As tagging receives increasing attention in both research and business communities, studies have found that users vary substantially in their tag usage, and suggested several factors that motivate user tagging. However, to date no quantitative study has assessed the strength of the effects of each motivation on levels of tag usage. This is surprising, since user participation is critical to the sustainability of content sharing communities, and a collaborative tagging system cannot succeed without higher level of user contribution. In what follows, we address this gap, by studying the strength of relationships between several motivations and users' tagging levels on Flickr, a prominent Web 2.0 photo sharing community. Currently, there are more than 35 million Flickr users, who have so far uploaded more 3 billion photos. Each Flickr user can upload images and make them viewable by self, by designated friends and family, or by all Flickr users. Flickr users can annotate images with tags - unstructured textual labels; and usually images are tagged only by the user who uploaded them.8 These tags make the images searchable by the uploading user, as well as by others. In addition, each user can designate other users as "contacts," people whose photos the user follows (contacts are often reciprocal). To understand what underlies tagging, we need to find out what motivates sharing in online environments, and in particular, what motivates tagging. Furthermore, we need to measure the degree to which different motivations affect tagging activity. While some studies has identified individual-level motivations for tagging, other studies have looked solely at the social level, focusing on the social presence as a driver of tagging.