We describe online collaborative communities by tripartite networks, the
nodes being persons, items and tags. We introduce projection methods in order
to uncover the structures of the networks, i.e. communities of users, genre
<br />To do so, we focus on the correlations between the nodes, depending on their
profiles, and use percolation techniques that consist in removing less
correlated links and observing the shaping of disconnected islands. The
structuring of the network is visualised by using a tree representation. The
notion of diversity in the system is also discussed.