  The number of people using online social networks in their everyday life is
continuously growing at a pace never saw before. This new kind of communication
has an enormous impact on opinions, cultural trends, information spreading and
even in the commercial success of new products. More importantly, social online
networks have revealed as a fundamental organizing mechanism in recent
country-wide social movements. In this paper, we provide a quantitative
analysis of the structural and dynamical patterns emerging from the activity of
an online social network around the ongoing May 15th (15M) movement in Spain.
Our network is made up by users that exchanged tweets in a time period of one
month, which includes the birth and stabilization of the 15M movement. We
characterize in depth the growth of such dynamical network and find that it is
scale-free with communities at the mesoscale. We also find that its dynamics
exhibits typical features of critical systems such as robustness and power-law
distributions for several quantities. Remarkably, we report that the patterns
characterizing the spreading dynamics are asymmetric, giving rise to a clear
distinction between information sources and sinks. Our study represent a first
step towards the use of data from online social media to comprehend modern
societal dynamics.
