  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.
