In 2010–2012, the free web lost *hundreds of thousands* of early adopters because we had no way to easily migrate from server to server; and lots of early server administrators closed down with little or no warning. This set the free web back at least five years, because you couldn’t trust your account and identity and friendships and content to exist tomorrow.
Interesting claim. I'd have guessed <100k and suspect causality also goes in other direction (disinterested users, shut down server). Still, I imagine the negative impact large. I imagine it will repeat with Mastodon.