Every day, around four million people fly on airplanes. This breaks down to roughly 1.7 billion people riding 25 million flights annually. 
Where the busiest flight routes are might surprise you, however. 
The outstanding Vizual Statistix tumblr pulled flight data from an Amadeus report about the top flight paths taken each year by passenger volume. 
The result will probably surprise you:
top flight routes
Visual Statistix