Unfortunately, as things stand, I don't think Roll20 usage stats are going to be very helpful. This is because Roll20 has yet to support any of the PF2 adventure path modules. And since the Pathfinder APs are a big reason many fans of Pathfinder play the game, Roll20's lack of support means that relatively few people playing PF2 are going to be doing it on Roll20.
A better gauge would be to look at stats for Fantasy Grounds, who have support for every PF2 AP module that's been released. Unfortunately, I don't have a Fantasy Grounds account, or access to their recent usage statistics, so I don't know how well PF2 is doing. But that would be the place to look.