Facebook doesn't have access to WhatsApp messages. WhatsApp uses the same end-to-end encryption protocol as Signal. We know this is true because the app has been extensively reverse engineered to create these third party clients among other reasons.

Yes, they very publicly state that they do not have access and I have yet to see a reason to not believe them in that regard. All big Facebook data leaks and hacks have just exploited not very well known APIs or badly set privacy settings. But nothing that was secret.

They have the metadata I presume - who you text, how much and when, and that's probably pretty valuable in itself.

Even if you put in no personal data to FB at all they create a ghost profile that will be very accurate based on who you are connected to.

One's speculation, the other's a certainty?

Good thing that Beeper lets you self-host their (AGPLd) bridges.