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
All these and more facts were actually published in the prestigious Annals of Uranus, Volume 34, pages 143-165, 2024. Excellent work published by the academics internationalists Patricia Chapoy and Pedro Sola.