Fair use and plagiarism are related, but they are two separate things. Especially when talking about the legalities of things, as we are here, it's vital to be clear and accurate about what specific legal issues are under discussion.

Facebook isn't claiming fair use to write academic papers about the books they're taking parts from. They're claiming fair use to feed them into LLM training. If that is a usage that is deemed to fall under fair use, then it won't require specific citation, even if it requires attribution in the more general sense (ie, crediting all the works you fed into your word-chipper), any more than you're required to cite specific passages when you're making a wholesale parody of a copyrighted work (also fair use) or writing a fanfic based on it (also fair use).