(Bloomberg Business) -- Foursquare, the mobile app that allows people to search forpopular locations, came together in 2009 in an unlikely venue:a cash-only watering hole tucked away in the Lower East Sideof New York City. As Dennis Crowley, a co-founder and thecompany's chief executive officer, explains, the Magician was"always the epicenter" of where his friends would meetup.

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A predecessor to Foursquare that Crowley built, calledDodgeball, allowed people to "check in" to their favoritelocations—telling their friends where they were and whether theyliked it. By using Dodgeball, Crowley and his peers kept theMagician buzzing. His go-to bar could be empty at 6 p.m.,then flooded by a dozen of his friends after one ortwo checked in.

After the success of Dodgeball, he envisioned an app that coulduse check-in data to do more. Foursquare evolved into an app forsearching and discovering venues that have been rated by locals.Around 50 million people a month now interact with Foursquare dataand products, Crowley says, and all because of a question thatpopped up over beer nights at a dive with friends.