The largest employer in March 2013 was United Airlines, which had 82,385 employees. The second largest was Delta Airlines, which had 73,395 workers in the same period.
The largest "low-fare" airline employer was Southwest Airlines and AirTran Airways, which merged into one company in 2011. The combined company employed 45,971 people in March 2013.
The full airline employment report can be read here.