User privacy is a challenging issue that must be addressed urgently in current online social networking (SN) sites. One of the fundamental problems associated with the issue is the lack of support of a user-centric approach to managing and sharing user profile information in current SN systems. In this paper we present a user-centric approach based on a credential system to enabling a user-controlled attribute (persona) sharing in online SN sites. Specifically we extend a Google-initiated open source project called OpenSocial, which provides a framework to support user attribute sharing between gadgets and online SN sites, in order to allow users to selectively share their attributes among online SN sites. This paper details the design and implementation of our extension.