DEFINE S.N.S.

(1) construct a public or semi-public profile within a bounded system
(2) articulate a list of other users with whom they share a connection, and
(3) view and traverse their list of connections and those made by others within the system. The nature and nomenclature of these connections may vary from site to site.

Source:
http://jcmc.indiana.edu/vol13/issue1/boyd.ellison.html#definition