How many people deleted their own Myspace account? If critical mass is hit, critical mass is hit and there's no turning back.

Still it's a lot different from yelling "I'm leaving!" and then do nothing. They now are exposed to the "competition", which is dangerous from Twitter's point of view, because now all it needs to lose those users is a bigger enough users base that attracts them into Mastodon, and the more users migrate, the faster the migration becomes. In electronics that would be called an avalanche effect.