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
They're just using very simple scrapers that don't have any knowledge about how the site operates. The simplest counter would probably be using Anubis on the web interface.
I wouldn't mind waiting 2-3 seconds when first loading the site and mobile apps would remain unaffected since they use the API.