in a social system individual actions have the potential to trigger spontaneous collective reactions. the way and extent to which the activity (number of actions-A) of an individual causes or is connected to the response (number of reactions-R) of the system is still an open question. We measure the relationship between activity and response with the distribution of efficiency, a metric defined as η = R/A. Generalizing previous results, we show that the efficiency distribution presents a universal structure in three systems of different nature: Twitter, Wikipedia and the scientific citations network. To understand this phenomenon, we develop a theoretical framework composed of three minimal statistical models that contemplate different levels of dependence between A and R. the models not only are able to reproduce the empirical activity-response data but also can serve as baselines or null models for more elaborated and domain-specific approaches. Due to humans' social nature, the actions of individuals hold the potential to trigger spontaneous collective reactions, leading to complex dynamics. In order to understand human collective behavior, it is necessary to find the laws that relate the individual actions to the collective response of social systems. This topic has received considerable attention and has been approached from several perspectives 1-4. From diffusion on networked systems, a field which studies the spread of diseases or information and the emergence of cascading phenomena 5-7 to virality, a property of certain pieces of information that generate a wide response in social systems 8-10. Other works focus on the Influence Maximization problem, taking advantage of the diffusion mechanisms to find a set of individuals that maximize the response 11-13. Alternatively, the field of control theory aims to steer the collective behavior of a system by controlling the activity of a few individuals 14, 15. Our goal in this work is to develop a theoretical framework that relates the number of actions performed by an actor (an agent or individual) embedded in a social system; that is, her activity (A), and the number of reactions that these actions trigger in her peers, or response (R). To relate these two magnitudes we generalize the efficiency metric (η = R A), introduced by Morales et al. in the context of Twitter 16 , to other social systems. We follow a well established modeling approach in social physics: explain the macroscopic properties of the system assuming the simplest microscopic interactions between the actors to extract the most fundamental laws 17-23. The macroscopic property in which we focus is the distribution of efficiency. We have used this metric to analyze three kinds of social systems of different nature: social networks, collaborative networks and citations networks. In particular, we have worked with 14 Twitter conversations around different issues in Spain, Turkey, Palestine, Argentina and Colombia, the editions of the English Wikipedia and the scientific citations data of authors f...