I think it would be extremely foolish to assume they don't.

I suspected that might be/is the case - but I also read their privacy policy and they claim to provide end to end encryption, not store your messages after your device downloads it and I haven't seen any ads. So I was trying to figure if they're lying about the encryption or FB doesn't get any data apart from my contact list which presumably isn't encrypted end to end.

It's a weird situation really.