Online social networks (OSNs) have experienced tremendous growth in recent years and become a de facto portal for hundreds of millions of Internet users. These OSNs offer attractive means for digital social interactions and information sharing, but also raise a number of security and privacy issues. While OSNs allow users to restrict access to shared data, they currently do not provide effective mechanisms to enforce privacy concerns over data associated with multiple users. In this paper, we propose a multiparty authorization framework that enables collaborative management of shared data in OSNs. An access control model is formulated to capture the essence of multiparty authorization requirements. We also demonstrate the applicability of our approach by implementing a proof-of-concept prototype hosted in Facebook.