Well, if Facebook have access to message content (and voice conversation data) then users need to seriously reconsider using this service in a professional context. The amount of handover and confidential information (over voice conversations) is quite staggering within the NHS.
Yeah a key difference is that telegram supports end to end encryption with secret chat, whatsapp however has no such function.
I remember in the old days on irc using a encryption addon in mirc even though the servers had encryption, the addon ensured client to client.