In this case in particular it looks like an acquihire.
Meta just saw two engineers actually execute on the joke about "building Facebook in a weekend" except that it then really took off in its target niche and generated a ton of press.
I don't doubt that they're interested in the AI aspect, but I suspect that a significant contributor was that they demonstrated competence right in the middle of Meta's wheelhouse so why not just grab these guys?
It's also part of their longer-term trend of buying or burying any company that starts to get any press as a social media site of note outside of major players where that hasn't been an option.
This is really it. At Meta's scale, even if it's an long-shot for a competitor to hurt them, it's worth turning those low odds into zero odds.
Yet Zuck can somehow argue with a straight face FB has competition (apparently they straight up used to delete links to competitors like Google+ at the time, and also the constant copying of Snapchat) and Hacker News can split hairs over trivial definitions like "wdym fb no competition? email exists" or whatever