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
I guess they mostly scrape it. To waste resources posting here they have to find a way to make money in doing so. They put bots posting on facebook because they think it increases user engagement. They dont want to increase engagement on lemmy (not that it would work…).