First, I need to point out that only 15k players of the 600k registered users of Roll20 participated in this survey. I know myself and my group didn't participate, but then again, none of us follow the Roll20 forums with any regularity.
I suspect FATE will see a bump in the next survey due to it's ENnies, as will the OSR (as I'll ensure my groups and readers of The Tavern that use Roll20) have their votes counted.
In any case, even getting 15k responses from players (and 25k for games - meaning GMs I suspect) is still pretty impressive. 4.1% of games run on Roll20 identify as AD&D, OD&D or OSR. Nearly 27% identify as Pathfinder, 3.5 at nearly 18% (so 45% are effectively playing 3x), 4e is around 11% and the new baby in the D&D era of games, D&D 5e comes in at a hair over 12%.
Over 70% (give or take, as more than one choice could have been chosen) of the games being run on Roll20 are D&D in heritage. Interesting numbers.