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One of the most compelling social questions, which until now was left unanswered by current technology, is "Who's that?" This question is usually asked about a new unfamiliar person in a social setting who piques your curiosity. This article presents WhozThat, a system that ties together online social networks with mobile smart- phones to answer this common and essential social question. WhozThat builds an infrastructure that shares social networking IDs locally, using wireless technology, while also leveraging a wireless connection to the Internet's social networks to bind identity with location. In addition, WhozThat offers an entire ecosystem on which increasingly complex context-aware applications can be built; that is, once the environment knows who one is, the environment can adapt its content based on the individual's identity or even the collective tastes of a local group. We describe a prototype of the basic WhozThat system and also describe a more advanced service we built, a context-aware music player. We also discuss security and privacy issues introduced by mobile social networks.