  We investigate the temporal patterns of human communication and its influence
on the spreading of information in social networks. The analysis of mobile
phone calls of 20 million people in one country shows that human communication
is bursty and happens in group conversations. These features have opposite
effects in information reach: while bursts hinder propagation at large scales,
conversations favor local rapid cascades. To explain these phenomena we define
the dynamical strength of social ties, a quantity that encompasses both the
topological and temporal patterns of human communication.
