  We study the relaxation response of a social system after endogenous and
exogenous bursts of activity using the time-series of daily views for nearly 5
million videos on YouTube. We find that most activity can be described
accurately as a Poisson process. However, we also find hundreds of thousands of
examples in which a burst of activity is followed by an ubiquitous power-law
relaxation governing the timing of views. We find that these relaxation
exponents cluster into three distinct classes, and allow for the classification
of collective human dynamics. This is consistent with an epidemic model on a
social network containing two ingredients: A power law distribution of waiting
times between cause and action and an epidemic cascade of actions becoming the
cause of future actions. This model is a conceptual extension of the
fluctuation-dissipation theorem to social systems, and provides a unique
framework for the investigation of timing in complex systems.
