It doesn't look like "they knew enough to choose software that could fundamentally scale". It looks more like they choose the first option that was available to

It doesn't look like "they knew enough to choose software that could fundamentally scale". It looks more like they choose the first option that was available to them. Apple also choose XMPP for Facetime as did a million other companies that serve similar scales of simultaneous clients. They may have hired people later on (though this entire article is kind of showing that no, they actually didn't hire that much technical people). But by the time they even hired the first Erlang engineer they had already won over the European market. And for the record it was not precisely due to the quality of either their Android or iOS android apps -- the J2ME and Symbian clients had quite a following and I would even bet were more used. Original Whatsapp was a disaster and this did not prevent them from gaining marketshare. You could basically login as anyone just by having their phone number (which maps to their JID) and defeating their XMPP obfuscation layer (some XML encoding, which was easily done due to the easily RE Java clients). It was only later on (2011ish?) that they started getting somewhat serious, which coincided with them, already swimming in users (and money?), starting to get more evil (with a more hardcore stance against 3rd party clients).