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 don’t see any way to “mitigate” this while still using the ActivityPub protocol. This isn’t about a bot posting on the Fediverse, it’s about reading the Fediverse. If you want to prevent that then you’re probably talking about some form of DRM or a walled garden.