  In social tagging systems, the diversity of tag vocabulary and the popularity
of such tags continue to increase as they are exposed to selection pressure
derived from our cognitive nature and cultural preferences. This is analogous
to living ecosystems, where mutation and selection play a dominant role. Such
population dynamism, which yields a scaling law, is mathematically modeled by a
simple stochastic process---the Yule--Simon process, which describes how new
words are introduced to the system and then grow. However, in actual web
services, we have observed that a large fluctuation emerges in the popularity
growth of individual tags that cannot be explained by the ordinary selection
mechanism. We introduce a scaling factor to quantify the degree of the
deviation in the popularity growth from the mean-field solution of the
Yule--Simon process, and we discuss possible triggers of such anomalous
popularity behavior.
