Look at the user figures for Lemmy instances and it becomes very clear that the number of people using Lemmy isn't even in the hundreds if thousands. So yes I'd say it is possible to estimate that less than 100k people made the switch.

I never use reddit unless someone sends me a link for something specific (that someone could be google) - I don't have their app installed and I never go to them naturally...I probably see 1 reddit post a week on average.