Obviously the best way to communicate is via carrier pigeons.
I'm saying this isn't needed and serious innovation. It's a popularity and standards game. We definitely didn't use carrier pigeons before Whatsapp. IM clients have been ubiquitous for years.
On desktop, not on mobile. This is the first big mobile IM service and the network effects are tremendous. Every other IM service is crappy (Skype) or late (Voxer) or crappy and late (Hangouts).