One of the points that came up from the #mastodonmeetup in Helsinki: there is a precedent for the kind of shift we are seeing from #Twitter to #Mastodon. When #Freenode #IRC went all bananas, people and projects started leaving in droves. Freenode is now completely empty.
Now the scale is different, of course, but it shows what can happen.
We are witnessing the biggest platform transition of our time. I'm still not sure if it's sustainable (it really depends on how #Mastodon can eat up the different people coming), but as a phenomenon it is incredible.