Now you can get 50 free United Mileage Plus miles for checking in at the airport with either FaceBook or FourSqaure. Just link your Mileage Plus number to either account and you’ll earn miles with every check in at United airports. Make sure to link you account here.
Also, here is a list of all the airports that qualify.
Finally, according to the terms of the agreement, you will receive emails with special offers from retailers at that airport. If you are sensitive to SPAM and do not like your email inbox cluttered by crazy promotions, I would steer clear of this promo. In that case 50 miles might not be worth your time.