As mobile devices become location-aware, it will become possible to know when people are physically co-located and to incorporate this information into social software. Is this valuable? A prototype social networking system based on physical co-presence was created and tested wizard-of-ozstyle at four different physical social events by providing event attendees a digital link back to others at the event. Usage of the system was higher than expected, suggesting a meaningful role for incorporating shared physical events into social networking software. Usage and questionnaire analyses suggest some guidelines for design of such systems. Author Keywords Social computing, social networking, events, mobile computing. ACM Classification Keywords H5.3. Information interfaces and presentation (e.g., HCI): Group and Organization Interfaces.