I'm not sure who is pushing #GlobalSwitchDay probably won't amount to anything, some of these platforms are certainly not ready a massive migration. The #RedditMigration was rough enough on Lemmy, it actually murdered #Kbin; #loops #PeerTube and #Friendica don't have nearly enough well-maintained instances to support any sort of massive migration. And Signal, well its better than WhatsApp, and its mostly Free Software, but it doesn't really align with all of the others that are, or at least intend to be in the case of Loops, federated services based on ActivityPub.
Reminds me of when Google Talk first rolled out as a service that utilized XMPP/Jabber, and then by the time their users made up like 90% of the xmpp comm unity, they stopped using the protocol, made it not interoperable with anything else, all but killed XMPP, turned Google Talk into Google Hangouts, than Google Duo, and I'm not sure where it ended but it too is a shell of its former self.