The same places they have been:
Facebook for the really old
TikTok for the kids
CoHost/Tumblr/Instagram/<insert gallery site> for the artists
Mastodon for the techies/furries and friends/nerds
Bluesky(eventually) for those who HAVE to have Twitter work exactly the same and can either mooch invites or wait on a huge waiting list/maybe techbros?
Musktwitter for Elon fanboys, because let's be real, they'll keep going until it literally doesn't exist
Discord for the communities that would be better suited by forums but refuse to admit that/friend groups that don't need to follow anyone else and actually are better served by nuIRC
Also Facebook is supposed to be tieing into the Fediverse(aka what Mastodon uses) BUT from everything I heard they're only going to ingest posts, but not actually send the replies out to servers still federated with them, so if you want to see some FB user said on your thread you would need to sign up on THEIR service. Basically a read-only connection to Fedi. Not that it makes a huge difference though, considering there's quite a few instances I know of planning to block FB's servers form talking with theirs as soon as it launches anyways, on general principle.