Using Facebook Messenger, WhatsApp, etc. is optional. Nobody has to use these platforms. So it's unlike having a government-mandated microphone and camera in your home or school, since you could not easily avoid those, and (in most democracies) you have a right to privacy in those places (well, in your home anyway).
If you want your messages encrypted, do it yourself. Then you are in control, not some big corp.
If you want your messages encrypted, do it yourself. Then you are in control, not some big corp.