Social influence drives both offline and online human behavior. It pervades cultural markets, and manifests itself in the adoption of scientific and technical innovations as well as the spread of social practices. Prior empirical work on the diffusion of innovations in spatial regions or social networks has largely focused on the spread of one particular technology among a subset of all potential adopters. Here we choose an online context that allows us to study social influence processes by tracking the popularity of a complete set of applications installed by the user population of a social networking site, thus capturing the behavior of all individuals who can influence each other in this context. By extending standard fluctuation scaling methods, we analyze the collective behavior induced by 100 million application installations, and show that two distinct regimes of behavior emerge in the system. Once applications cross a particular threshold of popularity, social influence processes induce highly correlated adoption behavior among the users, which propels some of the applications to extraordinary levels of popularity. Below this threshold, the collective effect of social influence appears to vanish almost entirely, in a manner that has not been observed in the offline world. Our results demonstrate that even when external signals are absent, social influence can spontaneously assume an on-off nature in a digital environment. It remains to be seen whether a similar outcome could be observed in the offline world if equivalent experimental conditions could be replicated.collective behavior | social networks | fluctuation scaling S ocial influence captures the ways in which people affect each others' beliefs, feelings, and behaviors. It has traditionally been within the domain of social psychology with a particular focus on microlevel processes among individuals (1), but it also plays a prominent role across the social sciences, for example in the study of contagion in sociology (2), herding behavior in economics (3), speculative bubbles in financial markets (4), voting behavior (5), and interpersonal health (6). Social influence plays an especially important role in cultural markets (7), for products such as books and music, and generally pervades any arena of life where the attitudes and tastes of individuals are influenced by others.It is often useful to distinguish between local and global sources of influence, which typically are identified with an individual's interpersonal environment and the mass media, respectively (8). The overall social influence arises from a mixture of local and global influences, which themselves emerge from different signals. The fact that these two processes operate at very different scales poses considerable challenges for the empirical study of social influence. For the purposes of our study, we define (i) local signal as information on the behavior of individuals who are friends or acquaintances of ego, the person whose behavior is being analyzed, and (ii) global signal as i...