Well they can if fb include a copy of the session key, encrypted with the public key of the escrow authority, appended to the ciphertext. The crypto is done by the fb app, so it is within their ability.
Big companies are easier to coerce than e.g. the pgp developers. There is no way for you to wrap your own encryption layer around the one used by WhatsApp/etc. You can post pgp messages on those systems but that is something very few will do.
