WhatsApp, arguably the most popular cross-platform messaging service out there today processed a record 18 BILLION messages on a single day on December 31st, 2013. That’s 7 Billion inbound and 11 Billion outbound messages.
If you’re wondering why the number of inbound and outbound messages is not the same, WhatsApp explains:
[...] difference in inbound vs. outbound is due to group chat. sending one msg into group chat of 10 people is 1:10 inbound:outbound
That is an interesting logic they follow there. To me, when a group message is sent out, a single message goes out but is instead is being delivered, independently, to multiple different devices and users. So that’s 1 outbound message but multiple inbound ones.
That said, that’s an insane amount of messages for a single day. Hats off to their engineering and ops team for this.