Happy 4sqDay! April 16, four-squared — get it? Today, Foursquare announced that its reached 20 million users and 2 billion check-ins! The site has grown rapidly, especially among Millennials who are constantly on their phones and seek to inform their friends of their whereabouts. In fact, according to our recent Ypulse research, 21% of collegians and 13% of high schoolers use the site to check in. (USA Today)