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 illuminate the need for having a precise conceptual apparatus that can grasp the complexity of contemporary online phenomena and their social dynamics. Exploring various accounts of SNS as part of the wider Web 2.0 realm, this paper approaches the widespread assumption that SNS bring forth a number of changes in the social as well as institutional arrangements surrounding their being used. Distinguishing between an instrumental and an institutional approach towards SNS, this paper suggests that contemporary research on SNS is roughly divided into two broad streams, one that focuses on how SNS are brought into service by users, and the other on how SNS bring users into service. The difference between these approaches is framed by suggesting a conceptual separation between individual-oriented and system-oriented agency. In order to overcome the difficulties attached to understanding the social dynamics of SNS as a distinct application within the Web 2.0 realm, it is argued that the term ‘social intermediaries’ offers a way to conceptualise SNS with respect to their functional position in the social realm, thus providing an important alternative to contemporary instrumental and institutional accounts. © 2012 Copyright Academy of Social Sciences.