At the end of the day, you’ve got to trust someone. I’m 200% convinced meta mines the social graph, of course they do, and provide access to law enforcement with a pro forma request. But I’m also 199% sure they don’t actually read your messages once unencrypted, reencrypts them and sends them as hidden payloads or does something else with it. The damage, should it be discovered, would be untold.
And while I don’t trust Meta on a lot of things, I know enough people there to realise that if they did that it would leak.