People were worried when Mastodon went from 20,000 to 500,000 people too. I don't see why you would think 1mil -> 2mil to be any different. I really don't agree with the comments that suggest that Mastodon shouldn't grow.
@naii Active users since 2.1.2 release: https://dashboards.instances.social/dashboard/db/network-activity?orgId=1 collected by @TheKinrar
Registered users growth: https://dashboards.mnm.social/dashboard/db/user-growth?orgId=1 collected by @eliotberriot