In old Foursquare, you could tap the map icon next to 'nearby' and 'worldwide' to see which of your friends were nearby at any given moment. Now you just have to tap on the 'Explore' tab to see an even prettier version of that map!
You'll see the friends nearby map at the top of the screen. Tap on the map to enlarge it, and zoom and tap the blue button to search another area. Tap on the map pins to see places that Foursquare recommends just for you, and pinch and pull to zoom in and out and search other areas.
Tap 'view map' at the bottom of the Explore page to see where your friends are, as well as the map pins of some recommended places. Pinch and pull to zoom in and out and swipe to move around and search different locations. Tap the blue button to refresh the results, then use the arrow at the bottom of the screen to scroll through the places represented by the map pins.