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Mobile social services enable increasingly popular forms of social interaction among mobile-device users, such as finding out when people of interest are nearby. However, today’s services raise important privacy concerns because they concentrate location information for many users under a single administrative domain. We present a privacy-preserving framework for these services in which each person maintains her own location history in her own Virtual Individual Server (VIS), a personal virtual machine running in a utility computing infrastructure. We enable location sharing among VISs through self-organizing overlay networks, one per social group with which VIS owners wish to share information. We describe an implementation of this framework that exploits skip graphs and Z-order space-filling curves to provide efficient and scalable operations on distributed location data. Finally, we demonstrate the feasibility of our approach by evaluating our implementation using real-world location traces gathered for this purpose. Our decentralized approach makes large-scale privacy breaches much less likely than in centralized architectures, and gives people fine control over what location information they share with whom.