United MileagePlus members can now earn 50 miles per day at participating airports for checking in with Topguest via Facebook or Foursquare. Register at http://www.mileageplus.topguest.com and choose to connect with Facebook or Foursquare. Check-in with your smartphone or mobile browser when you’re at the airport and you’ll earn 50 miles and receive special offers from nearby retailers, car rental agencies and restaurants.