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.
We need to be able to sort of culturate the influx, I think.
We can see historical breakdowns in usenet's eternal september.
I can't help but wonder what the critical mass is one way or the other
I think because mastodon is /so/ distributed though, it has a much higher critical threshold before disintegration kicks in.