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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
If they have a brain, and they do have the experience from Threads, they don't need to scrape Lemmy. They can just set up a shell instance, subscribe to Lemmy communities, and then use federation to get their data for free. That doesn't use robots.txt at all regardless.