In recent years the development of a multitude of environments for social interaction has provided unprecedented opportunities for mass participation in social activities on a global scale. Participation in social action can take many forms from well-structured dialogues such as those taking place in scientific conferences to spontaneous crowd reactions similar to those occurring in sports or mass entertainment venues. Our research focuses on the creation of methods for the expression, rendering and analysis of collective reactions in social activities. Collective in this context refers to a number of reactions with similar content, referring to the same situation and posted by a significant number of people at approximately the same point in time. We examine the types of such reactions and propose a number of rendering methods that take into account their magnitude, persistence and the aesthetics of the environment they appear in. We also describe analysis tools for tracking the emergence and evolution of such phenomena and discovering their causes. Finally, we propose methods by which the results of this analysis can be used in the creation of richer and more engaging social interaction experiences.