I haven't had very low loads on UA. However, back in 1987 my wife and I were the only passengers in one portion of a cabin on an LH flight from ORD to FRA... We were offered dinner right after takeoff, and tucked into the middle sections for sleeping... and woken up for breakfast just before arrival... Never had it that empty again... Though I must say that flying AF from Paris to Nairobi during Gulf War I was quite empty (probably 10% load)...