Depends. If an instance scales up by 100x overnight it'll probably go offline. But the nice thing is that no single instance has to scale 100x to scale the net

Depends. If an instance scales up by 100x overnight it'll probably go offline. But the nice thing is that no single instance has to scale 100x to scale the network by 100x. 100 instances can scale 2x or 1000 instances can scale 1.2x or 10000 instances can scale 1.02x... You get the point. Mastodon is designed to have people sign up into communities and then spread out into the fediverse from there.