While the drop off is interesting by itself, I think you lose part of the story by not comparing the active users over time with other popular F2P games. What I want to know is, is a 30% drop-off within the first month of release unusual? Is that small or large in context? I have no idea.
The only problem with investigating this question is that there’s really no other game like Pokemon Go to compare it to. The closest I’m aware of is Ingress, but that doesn’t have nearly the same historic brand-loyalty associated with it. I still think it would be a useful comparison, though.