There's a concept called Embrace, Extend, Extinguish (seemingly coined, in that form, in a Microsoft antitrust lawsuit). Here's the Wikipedia page on it: https://en.m.wikipedia.org/wiki/Embrace,_extend,_and_extinguish

As I understand, people argue that Facebook/Meta, via Threads, will use this strategy in the long-term to either kill, or make effecitvely obsolete, the open technology behind Mastodon. If not that, then they could easily make the federation part of Threads buggy & unreliable, souring their users' opinions on the "fediverse".

They don't need to control anyone; they only need to host a majority of the userbase (by being the most popular federated site). And they're not starting from a user count of 1 or 10, unlike a lot of Mastodon sites.

Obviously, Mastodon & Lemmy, and the sites that run them, can keep chugging along just fine, but it's argued that if Meta makes their federation implementation sub-par (or otherwise sabotages it), it'll hurt the user-base growth of sites that use these projects (as people will see begin to see it as unreliable or what-not).

Is it as doom and gloom as people make it seem? Idk, I haven't had time to care.