There have been many location sharing systems developed over the past two decades, and only recently have they started to be adopted by consumers. In this paper, we present the results of three studies focusing on the foursquare check-in system. We conducted interviews and two surveys to understand, both qualitatively and quantitatively, how and why people use location sharing applications, as well as how they manage their privacy. We also document surprising uses of foursquare, and discuss implications for design of mobile social services. Author Keywords foursquare, mobile computing, social computing, check-in, privacy, location based service, uses and gratifications ACM Classification Keywords H.5.m Information Interfaces and Presentation (e.g., HCI): Miscellaneous General Terms Human Factors, Design