  Collaborative tagging has been quickly gaining ground because of its ability
to recruit the activity of web users into effectively organizing and sharing
vast amounts of information. Here we collect data from a popular system and
investigate the statistical properties of tag co-occurrence. We introduce a
stochastic model of user behavior embodying two main aspects of collaborative
tagging: (i) a frequency-bias mechanism related to the idea that users are
exposed to each other's tagging activity; (ii) a notion of memory - or aging of
resources - in the form of a heavy-tailed access to the past state of the
system. Remarkably, our simple modeling is able to account quantitatively for
the observed experimental features, with a surprisingly high accuracy. This
points in the direction of a universal behavior of users, who - despite the
complexity of their own cognitive processes and the uncoordinated and selfish
nature of their tagging activity - appear to follow simple activity patterns.
