[nickjj]

Yeah that's true, in the Pidgin case you were still signing up for each service independently. Pidgin let you communicate through its app instead of the service provider's app. None of the service providers had data about the others.

I'm not sure how they plan to make things fully interoperable. Wouldn't that mean every chat app would need a complete chat history for every user multiplied by every chat app out there? Or instead they introduce a shared parent company / govt. entity where all chat messages are sent through, stored and pushed to users. Time to bust out the tin foil hats!

[Cerium]

I think you could make it work with only some username namespace. Eg: whatsapp.cerium vs fb.cerium. Facebook would forward messages to whatsapp usernames and vice versa.