The very first version of foursquare (January 2009) was built as a “leaderboard for Saturday night” – a way to motivate your friends to go out and explore the real world and a means to visualize your adventurousness. Points for check-ins, badges for accomplishments, and a leaderboard to compare the results with your friends all fed into this idea. At foursquare, we’re big believers in the power of software to encourage and motivate behavior

viaIt all started with a check-in: the vision for #4sq3 and beyond | Foursquare Blog.