I'm not worried at all about Zuck taking over Mastodon at all, they'll try but they are just so incompetent, because literally every single product idea they have they either stole or bought from somebody else. Great tech, terrible products, zero originality is the Facebook mantra and that is because they have a delusional CEO that they can't fire, because Zuck has delude himself into thinking he's an "ideas" guy like Jobs instead of an "executions" guy like Bezos that he really is, and until he realizes that, he will always fail.
(also, delusional for actually thinking Ready Player One is a good book)
If making a TikTok clone didn't get people to switch from TikTok, why would they think making a Twitter clone is going to get people to switch from Twitter?
The only way I see Facebook being a threat is when they give up on making their Twitter clone and start providing easy subscription service hosting for Mastodon/Lemmy to EEE. THAT would be the time to worry.