This special issue considers the promises of social media based forecasts as well as the perils and pitfalls it is plagued with, and strategies to address these problems. Its intended audience are computer scientists, social scientists, economists, statisticians, and other researchers interested in the application of multidisciplinary approaches to exploit user-generated contents to better understand (and predict) societal behaviors. This issue includes three works approaching such topics from different points of view: the credibility of data appearing in social media; the detection of unexpected phenomena as deviations of the "pulse" of social media; and a conceptual framework to survey the current body of research.