  The subject of collective attention is central to an information age where
millions of people are inundated with daily messages. It is thus of interest to
understand how attention to novel items propagates and eventually fades among
large populations. We have analyzed the dynamics of collective attention among
one million users of an interactive website -- \texttt{digg.com} -- devoted to
thousands of novel news stories. The observations can be described by a
dynamical model characterized by a single novelty factor. Our measurements
indicate that novelty within groups decays with a stretched-exponential law,
suggesting the existence of a natural time scale over which attention fades.
