Now that X got banned in Brazil and to potentially lose over 100M+ users we are starting to see which platforms they are choosing to sign up to.
So far, Bluesky is seeing a surge in user registrations after the invite system was lifted a year ago. I would expect Threads to also see a surge in registrations as well.
Mastodon however appears not to be even considered as a migration path at all yet, but either way it is still early days for all options.
We'll see in the next 6 months after this comment: https://news.ycombinator.com/item?id=39471807