  We develop a theoretical framework for the study of epidemic-like social
contagion in large scale social systems. We consider the most general setting
in which different communication platforms or categories form multiplex
networks. Specifically, we propose a contact-based information spreading model,
and show that the critical point of the multiplex system associated to the
active phase is determined by the layer whose contact probability matrix has
the largest eigenvalue. The framework is applied to a number of different
situations, including a real multiplex system. Finally, we also show that when
the system through which information is disseminating is inherently multiplex,
working with the graph that results from the aggregation of the different
layers is flawed.
