  In the era of social media, every day billions of individuals produce content
in socio-technical systems resulting in a deluge of information. However, human
attention is a limited resource and it is increasingly challenging to consume
the most suitable content for one's interests. In fact, the complex interplay
between individual and social activities in social systems overwhelmed by
information results in bursty activity of collective attention which are still
poorly understood. Here, we tackle this challenge by analyzing the online
activity of millions of users in a popular microblogging platform during
exceptional events, from NBA Finals to the elections of Pope Francis and the
discovery of gravitational waves. We observe extreme fluctuations in collective
attention that we are able to characterize and explain by considering the
co-occurrence of two fundamental factors: the heterogeneity of social
interactions and the preferential attention towards influential users. Our
findings demonstrate how combining simple mechanisms provides a route towards
complex social phenomena.
