Social interfaces are the way a system facilitates social behaviour between users.
Social interface refers to the way a system makes it possible for people to engage in social behaviour like communicating with each other. The social interface of a web app or networking site defines how the application can be used to interact with other humans.
"Social interface design" supports the view that social aspects of applications are increasingly important for how those are used and how successful they are in regards to user acceptance. The term itself is not yet broadly established in the ux-field, but has gotten more and more widely used over the last years (2005 - ). "Social interface" as a term stems from social sciences (see http://en.wikipedia.org/wiki/Social_interface)
Questions with this tag deal with social interfaces in general, not just social networking sites. If user experience concerns of a question inherently have to do with the way in which humans communicate and socially engage using an application or system, this tag is appropriate to use.
- http://www.joelonsoftware.com/articles/NotJustUsability.html - article explaining some ideas behind the term "social interfaces", Joel Spolsky, September 06 2004