  We present new empirical evidence, based on millions of interactions on
Twitter, confirming that human contacts scale with population sizes. We
integrate such observations into a reaction-diffusion metapopulation framework
providing an analytical expression for the global invasion threshold of a
contagion process. Remarkably, the scaling of human contacts is found to
facilitate the spreading dynamics. Our results show that the scaling properties
of human interactions can significantly affect dynamical processes mediated by
human contacts such as the spread of diseases, and ideas.
