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April 8, 2011, New York City: Foursquare has redefined what it means to "check in." More than 8 million Foursquare members worldwide use the two-year-old company's free mobile app to broadcast their location on social networks like Facebook and Twitter. In exchange, members get loyalty rewards or special offers from businesses. Foursquare doesn't charge merchants to use its platform -- yet. But with 500 million check-ins over the past year alone, Foursquare has already attracted partners like American Express and Starbucks. Is the Fortune 500 in its future?

Left to right: Bryce Roberts, managing director, O'Reilly AlphaTech Ventures; Ben Horowitz, partner, Andreessen Horowitz; Dennis Crowley, co-founder and CEO, Foursquare; Naveen Selvadurai, co-founder, Foursquare; Albert Wenger, partner, Union Square Ventures

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Last updated May 24 2011: 6:41 PM ET
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