In the introduction for this special issue we briefly review the development of social software, contrasting it with earlier group-based computer applications like groupware and computer-supported cooperative work. The contributions selected span a wide range starting from reflections on informality as a key property of social software. Case studies give details on using wikis for knowledge management and the influence of choosing a specific wiki engine. The transformation problem for non-digital information services is discussed in the context of a large research institute. More recent types of social systems and their potential use for knowledge management are discussed for microblogs as well as semantic wiki systems. Finally, an overview studies gives information on the intensity of social software usage for customer communications in large European companies.