Interesting article on tagging and classification challenges in social systems:
Collaborative tagging systems are powerful tools for organizing, browsing and publicly sharing personal collections of resources on the World Wide Web. They have enjoyed widespread adoption by end-users.
Collaborative tagging produces aggregations of user metadata, often referred to as folksonomies. These user-generated classifications emerge through bottom-up consensus by users assigning free form keywords to online resources for personal or social benefit. Del.icio.us, Flickr, 43things, Furl and Technorati are examples of web-based collaborative systems for building shared databases of items. The users of these systems create a flat metadata vocabulary that can be used to perform metadata driven queries, to monitor change in areas of interest or to discover emergent trends, such as the hottest/most popular topics in the system. In the past, folksonomies have often been seen as orthogonal to taxonomies and controlled vocabularies: the latter being rigid, hierarchical and organically hand-crafted by professionals a priori; the former being flat, inclusive and emerging from bottom-up users' input and consensus .