About 1,600 people worked at Foursquare over 17 years. Last month every one of them found out their stock was worth exactly $0, and most got the news in an email. If you had a smartphone in 2010, you probably used it. You'd check into a coffee shop or a bar and fight your friends to become its mayor. It got so hot that Yahoo and Facebook both came around with offers reported north of $100M. The photo Dennis posted is the wall from their first real office. Every new hire wrote their name in…