Emily Woolf, far right, uses the site Foursquare, which is available in 31 American cities, to find her friends when she wants to meet.Credit...Yana Paskova for The New York Times Twitter and Facebook ask users to answer the question: What are you doing right now? But for many urbanites in their 20s and 30s, two other questions are just as important: Where are you, and can I come join you? For the
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