Social media have transformed the Web into an interactive sharing platform where users upload data and media, comment on, and share this content within their social circles. Each content item is associated with an abundance of metadata and related information such as location, tags, comments, favorites and mood indicators, access logs, and so on. At the same time, all this information is implicitly or explicitly interconnected based on various properties such as social links among users, groups, communities, and sharing patterns. These properties transform social media into data sources of an extremely dynamic nature that reflect topics of interests, events, and the evolution of community opinion and focus. Social media processing offers a unique opportunity to structure and extract information and to benefit multiple areas ranging from computer vision to psychology and marketing. The objective of this special section is to provide an overview of the current research in emerging topics related to applications where social media can act as sensors of real-life phenomena and case studies that reveal valuable insights usually not possible with existing, limited, controlled, and laboratory-based datasets.