Comment by lukeschlather

It seems obvious that they can. It's my understanding for FB Messenger that the private key is stored encrypted with a key that is derived from the user's password. So it's not straightforward, but Meta is obviously in a position to grab the user's password when they authenticate and obtain their private key. This would probably leave traces, but someone working with company authorization could probably do it.

For WhatsApp they claim it is like Signal, with the caveat that if you have backups enabled it works like Messenger. Although interestingly if you have backups enabled the key may be stored with Apple/Google rather than Meta, it might be the case that with backup enabled your phone vendor can read your WhatsApp messages but Facebook cannot.