We study the semantic assortativity in the social networks hosted by the Flickr folksonomy, based both on the contact data and on the group membership data provided by the users. The social network built this way are complex one. Besides, one observes a clear assortativity pattern, stronger than in a suitable null model adopted for a comparison. Nevertheless, such semantical similarity does not appear to develop during the community evolution, but is rather the result of a pre-existing shared background between users.

We analyze a large-scale snapshot of del.icio.us and investigate how the number of different tags in the system grows as a function of a suitably defined notion of time. We study the temporal evolution of the global vocabulary size, i.e. the number of distinct tags in the entire system, as well as the evolution of local vocabularies, that is the growth of the number of distinct tags used in the context of a given resource or user. In both cases, we find power-law behaviors with exponents smaller than one. Surprisingly, the observed growth behaviors are remarkably regular throughout the entire history of the system and across very different resources being bookmarked. Similar sub-linear laws of growth have been observed in written text, and this qualitative universality calls for an explanation and points in the direction of non-trivial cognitive processes in the complex interaction patterns characterizing collaborative tagging.