Two parties communicating with emails does not need to register with same <mail service provider>.
But messaging (whatsapp, Telegram, gTalk, skype, facebook messaging, line, etc) is different.
Both parties should register with same <messaging service provider>
There is a high need to have an interworking mechanism.
eMail id is unique. messaging id too should be unique.
say : 1234@whatsapp; John@gTalk; 1234@viber; John@yahoo; 1234@line; john@facebook
Simialr to eMail protocols, we need mechanism to deliver text/voice/videos from one <messaging service provider> to other.
End user life becomes easier (by not registering every possible messaging service provider) with increasing messaging systems.