  User-generated content is shaping the dynamics of the World Wide Web. Indeed,
an increasingly large number of systems provide mechanisms to support the
growing demand for content creation, sharing, and management. Tagging systems
are a particular class of these systems where users share and collaboratively
annotate content such as photos and URLs. This collaborative behavior and the
pool of user-generated metadata create opportunities to improve existing
systems and to design new mechanisms. However, to realize this potential, it is
necessary to understand the usage characteristics of current systems. This work
addresses this issue characterizing three tagging systems (CiteULike, Connotea
and del.icio.us) while focusing on three aspects: i) the patterns of
information (tags and items) production; ii) the temporal dynamics of users'
tag vocabularies; and, iii) the social aspects of tagging systems.
