Why? From Who?
From everyone like right now where there’s a bunch of bugs and wants to have certain tools for moderation. Just look at how many issues have been added to lemmy in the past 3 weeks. And that’s just a fraction of Reddit users joining. Having millions of Facebook users being able to interact with lemmy will likely expose many more bugs and show the need for more moderation.
Same applies to your question about the troll wave - they are currently coming from Reddit. And once Meta joins - more fediverse exposure will let more people know of fediverse, and those new people will have trolls among them.
What are the chances that this is something so significant that people would be willing to switch software over it?
Meta has been in the market of attracting people for a long time. Don’t underestimate their R&D . People on Reddit used to join subs just for having certain bots. Meta can easily bring a ChatGPT bot to lemmy for example. Again this might not attract you but you have to think about the average person.
I think your one good argument against my scenario is defederation. Unlike XMPP the fediverse is already gaining critical mass where defederation can be a viable option. But again I’m sure Meta R&D can “help” with it