  The overwhelming success of the web 2.0, with online social networks as key
actors, has induced a paradigm shift in the nature of human interactions. The
user-driven character of these services for the first time has allowed
researchers to quantify large-scale social patterns. However, the mechanisms
that determine the fate of networks at a system level are still poorly
understood. For instance, the simultaneous existence of numerous digital
services naturally raises the question under which conditions these services
can coexist. In analogy to population dynamics, the digital world is forming a
complex ecosystem of interacting networks whose fitnesses depend on their
ability to attract and maintain users' attention, which constitutes a limited
resource. In this paper, we introduce an ecological theory of the digital world
which exhibits a stable coexistence of several networks as well as the
domination of a single one, in contrast to the principle of competitive
exclusion. Interestingly, our model also predicts that the most probable
outcome is the coexistence of a moderate number of services, in agreement with
empirical observations.
