Resumen de Evolution and ecology of the digital world: A complex systems perspective
"[eng] Online social networks (OSNs) enable researchers to study the social universe at a previously unattainable scale. The worldwide impact and the necessity to sustain their rapid growth emphasize the importance to unravel the laws governing their evolution. We present a quantitative two-parameter model which reproduces the entire topological evolution of a quasi-isolated OSN with unprecedented precision from the birth of the network. This allows us to precisely gauge the fundamental macroscopic and microscopic mechanisms involved. Our findings suggest that the coupling between the real pre-existing underlying social structure, a viral spreading mechanism, and mass media influence govern the evolution of OSNs. The empirical validation of our model, on a macroscopic scale, reveals that virality is four to five times stronger than mass media influence and, on a microscopic scale, individuals have a higher subscription probability if invited by weaker social contacts, in agreement with the ""strength of weak ties"" paradigm. 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. 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. In addition, we discuss the impact of heterogeneity in network fitnesses induced by competition between an international network, such as Facebook, and local services. To this end, we construct a 1:1000 scale model of the digital world, consisting of the 80 countries with the most Internet users. We show how inter-country social ties induce increased fitness of the international network. Under certain conditions, this leads to the extinction of local networks; whereas under different conditions, local networks can persist and even dominate the international network completely. Finally, we investigate how multiple coexisting networks, which form a so called multiplex system, facilitate search and navigation with only local knowledge. This task is especially important in decentralized architectures. In particular, we show that multiplex systems are not random combinations of single network layers. Instead, they are organized in specific ways dictated by hidden geometric correlations between the individual layers. We find that these correlations are strong in different real multiplexes, and form a key framework for answering many important questions. Specifically, we show that these geometric correlations facilitate: (i) the definition and detection of multidimensional communities, which are sets of nodes that are simultaneously similar in multiple layers; (ii) accurate trans-layer link prediction, where connections in one layer can be predicted by observing the hidden geometric space of another layer; and (iii) efficient targeted navigation in the multilayer system using only local knowledge, which outperforms navigation in the single layers only if the geometric correlations are sufficiently strong. Interestingly, many real systems fulfill these conditions. To conclude, from a system-level perspective, a prospering future in the digital age comprised of a diverse digital landscape with interacting, decentralized architectures is possible, but so is the opposite. It remains a task for society to create sufficient awareness and the correct incentives to create this future we desire. [spa] Esta tesis está dedicada a los retos de un mundo interconectado que ha emergido a partir de la reciente revolución digital en el que servicios digitales se desarrollan y compiten en la ausencia de control central. Por tanto, herramientas, ideas y técnicas de análisis de sistemas complejos son útiles y especialmente adecuadas para describir la evolución y competencia entre redes sociales online. El éxito del Internet ha conectado individuos a escalas sin precedentes y la Web 2.0 promociona hoy en día colaboración global y el intercambio de ideas casi instantáneo. Sin embargo, la dominación de unos pocos poderosos monopolios de información representa un peligro para la libertad de ideas y decisiones de individuos. Por tanto, dos factores son esenciales para un futuro próspero en la era digital: diversidad digital y decentralización. En cuanto al primero, hemos introducido modelos basados en observaciones empíricas que permiten entender mejor la dinámica y las interacciones competitivas de las redes sociales online, los sistemas claves en el cosmos de la Web 2.0. En particular, nuestros descubrimientos revelan las condiciones en las cuales la diversidad digital se puede sostener. Con respecto al segundo, el diseño de arquitecturas descentralizadas contiene retos específicos, entre los que nos dirigimos especialmente a la búsqueda y navegación basada exclusivamente en conocimiento local. Hemos revelado en qué condiciones la existencia de muchas redes interaccionando facilita estas tareas. Afortunadamente, muchos sistemas reales cumplen estas condiciones. Para concluir, desde una perspectiva a nivel de sistema, un futuro próspero en el mundo digital compuesto por un paisaje digital diverso con arquitecturas descentralizadas en constante interacción es posible, pero no seguro. En esta situación, la conciencia, así como la creación de los incentivos adecuados, son retos importantes que nuestra sociedad debe afrontar. Crear conciencia suficiente e incentivos correctos para crear ese futuro sigue siendo un reto para la sociedad. "
