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.
It was LibreSignal. That was a small scale but public meltdown from Signal's founder, however. "Signal" is already a common term and as such hard to claim sole rights over.
In the same github thread he, Moxie Marlinspike, also insisted that LibreSignal abstain from using Signal's servers, which sort of defeats the purpose of a third party app. Signal's initial federation with other servers was rolled back, which again paints an image of a budding data silo.
You're right, Molly seems to have navigated the third party relationship better, perhaps because Marlinspike has been replaced with new leadership as well?
My ¢2 is this: At this point, with tech leaders making hard right political turns, or just idiosyncratic attacks like Wordpress' Mullenweg, trust in centralised services should be at a minimum.
Signal already foisted a cryptocurrency (that, incidentally, Marlinspike had consulted on) on their users. Controversial self-serving isn't new to the organisation, and with full control of a significant share of the private/secure IM market, it's only a matter of time before their stewardship veers of track.