> they have no access to the text of the messages due to E2EE.
Correction: they might not have access to the message text. It's entirely possible (if not plausible: FB doesn't exactly have a good track record) for FB to just self-MitM the E2EE and see everything that passes through their servers.
From their site:
> The verification process is optional for end-to-end encrypted chats, and only used to confirm that the messages and calls you send are end-to-end encrypted.
Even this process--which I'm sure very few people do--is fallible given the lack of authenticity: there's no way to confirm that the given keys are what's actually used for encryption.
Yes, this may come across as very "tinfoil-hat-y," but do you really trust FB to not be exploring every possible avenue to increase their data streams?