Over recent years significant changes in the nature of online communication have taken place, not the least because of the emergence of Web 2.0 and the subsequent proliferation of Social Network Sites (SNS). These changes have provided researchers, scholars and critics with a multi-levelled field of investigation while at the same time illuminating the need for having a precise conceptual apparatus which bears a possibility to account for the social dynamics of contemporary online phenomena. This paper explores the possibility to understand the various appearances of Web 2.0 and SNS as bringing forth a number of changes in the social as well as institutional arrangements surrounding their being used. In overall terms, these phenomena are conceptualised from a perspective that either focuses on individual instrumentality or institutional exploitation. Whereas the instrumental view primarily locates agency at the level of individual users and the personal benefits associated with the performance of various technologically mediated actions, the institutional view ascribes agency to the Web 2.0 applications which are assumed to commercially deploy their users as objects of inquiry and sources of information. In this paper, these ideas are further elaborated by suggesting a conceptual separation between individual- oriented and system-oriented agency that lays the foundation for developing a conceptual apparatus that accounts for the social dynamics of Web 2.0 and SNS. Taking the different ways of locating agency as a point of departure, it is furthermore suggested that the term ‘social intermediaries’ provides a possibility to conceptualise Web 2.0 and SNS from the perspective of their functional position in the social realm, thus providing an important alternative to contemporary instrumental and institutional accounts. Drawing on Georg Simmel’s understanding of sociality, the notion of social intermediaries is conceptualised as a ‘third Other’ that is assumed to intervene in the social realm and the processes as well as practices of of which it consists.