That system was the backend of the functionality currently known as Facebook Messenger, not WhatsApp.
One might note, snidely [1], that three-four years after freezing development of an Erlang-based messaging system, starving maintenance work on that system of engineering effort, devoting massive engineering resources to a from-scratch C++ rewrite, and in the meantime blaming the language for relatively minor system design issues that could have been improved with a fraction of the effort (including, but not limited to, Erlang's ability to wrap allegedly critical C++ components) … Facebook plowed $19B into the acquisition of an Erlang-based messaging system.
[1] as a main author of the Facebook Chat version written in Erlang
> after freezing development of an Erlang-based messaging system...
> Facebook plowed $19B into the acquisition of an Erlang-based messaging system.
They didn't care what it was written in. They would've spent it on Whatsapp even it was written in PHP (like Slack). They paid for users and market penetration.
One might note, snidely [1], that three-four years after freezing development of an Erlang-based messaging system, starving maintenance work on that system of engineering effort, devoting massive engineering resources to a from-scratch C++ rewrite, and in the meantime blaming the language for relatively minor system design issues that could have been improved with a fraction of the effort (including, but not limited to, Erlang's ability to wrap allegedly critical C++ components) … Facebook plowed $19B into the acquisition of an Erlang-based messaging system.
[1] as a main author of the Facebook Chat version written in Erlang