If you’re interested in the growth and use of tagging, or folksonomy generally, take some time to read The Structure of Collaborative Tagging Systems (PDF). The paper discusses what people tag (using data from Del.icio.us), the purpose of tagging (“fundamentally about sensemaking”), some of the problems of tagging (in particular polysemy and synonymy), and compares tagging to taxonomy.
One aspect I found interesting was the dynamics of tagging. In Del.icio.us, individuals users tag URLs how ever they like. When you look at all the tags of any given URL you “might expect that individuals’ varying tag collections and personal preferences, compounded by an ever-increasing number of users, would yield a chaotic pattern of tags”. The paper reports otherwise. A stable pattern quickly emerges, which implies a consensus is formed after as little as 100 tags. The authors suggest imitation and shared knowledge between users is the cause of this. There may be a link here with The Wisdom of Crowds, which is still in my bought-but-not-read pile.