I usually fall when I'm attempting to recover, or over-correcting, from some sort of mishap; means I'm not usually going full speed. And very rarely, from catching an edge.
But it depends mostly on how soft the snow is. Lots of powder means I'm going to be going fast and taking chances and falling a few times every run. Early season / hard-pack day and I'm only falling a few times a day, knock on wood. :D
Not falling, and being a good rider are two different things; not to be confused.