Years ago I flew a Braniff flight (727) from Chicago to DFW, the plane went on to Acapulco. We had five flight attendants and two passengers in coach (a honeymoon couple) and me in first. They moved the honeymoon couple up to first.

I worked for USAir and we had one flight from Trenton to Pittsburg with nobody on it! We also had a commuter that went from Trenton to Atlantic City, often there was nobody on that leg. Several times I went on that leg and was the only passenger on the plane.