I have never been the only person on a flight. I went on my first TATL trip, almost 2 years ago. Flew on LX from ZUR-CDG and FCO-ZUR (took trains from CDG to FCO), both on A319's. I'd say that on both flights, the plane was less than half full.

I had flights with US just over a year ago. Looked on the seat map for those flights. There were 100+ seats empty on a 757. That was 2 or 3 weeks in advance of that date. Somehow in those 3 or 4 weeks, that flight got full.