Obviously a few years ago, the API changes caused the reddit community mods to strike, and caused a mass-blackout of most reddit core communities. Eventually Reddit removed a handful of mod teams on some notable subreddits and caused the rest to chicken-out. But it did birth the Fediverse properly.
I suspect Reddit will make another step at some point which causes another comparable exodus. This time, if they do, the Fediverse is far better developed to handle it. What do you imagine it might be?
As far as I know, lemm.ee was the only notable instance that was shut down - and I believe most communities on there successfully migrated to a mixture of piefed.social, lemmy.zip and a smattering of others.
Yes, our community has had the bad luckof having had to move three times. Shedding members each time. The last move was due to the sudden shutdown of lemm.ee.
Oh collapse. I also had to move, and rebuilt well enough.