If you only focus on sending messages, you will get huge scalability. If you look at it, much of the problem was already solved at a fundamental level: kernel/

If you only focus on sending messages, you will get huge scalability. If you look at it, much of the problem was already solved at a fundamental level: kernel/network developers have been thinking about how to efficiently send messages since forever. Jabber devs had thought about certain problems on top of that as well. Put those together and don't add anything, you should be able to send a huge number of messages. Keep in mind it's not latency sensitive either, people can live with getting their messages 1s or late and with significant jitter. That's not to poop on the success of WhatsApp, I do think it's a good product. But it's a the product that enables the engineering to stay focused.