Dropsitenews published a list of websites Facebook uses to train its AI on. Multiple Lemmy instances are on the list as noticed by user BlueAEther
Hexbear is on there too. Also Facebook is very interested in people uploading their massive dongs to lemmynsfw.
Full article here.
Link to the full leaked list download: Meta leaked list pdf
The point they're making is that they don't need to scrape the data. It is available via federation. Scraping the data is less efficient and can negatively affect the platform performance, versus the built in federation system where that data sync is intentional.
Especially when Meta has a fediverse presence. The reason they're scraping is likely because instances have blocked theirs, in part to prevent this exact thing.
They could just spin up a no-name instance that isn't associated with them to get it through federation, though. It still doesn't make sense to scrape.
They'd have to host it from somewhere not related to Meta in any way, otherwise someone on the fediverse would find that link and spread the word, and it would be blocked the exact same way. It only takes one person making that connection, Meta knows they're hated.