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WhatsApp is working on cross-platform messaging
(www.theverge.com)
This is a most excellent place for technology news and articles.
The interesting bit here is if and how that'll allow non-whatsapp users to be added to whatsapp group chats. 1:1 communication already works outside of whatsapp (worst case via SMS), but they control those group chats.
The interesting part will be adding WhatsApp users to actually trusted private messaging apps such as literally anything else.