This paper explores the application of role-based access control to social networks, from the perspective of social network analysis. Each tie, composed of a relation, a sender and a receiver, involves the sender's assignation of the receiver to a role with permissions. The model is not constrained to system-defined relations and lets users define them unilaterally. It benefits of RBAC's advantages, such as policy neutrality, simplification of security administration and permissions on other roles. Tie-RBAC has been implemented in a core for building social network sites, Social Stream.