Comment by toast0

Nit: WhatsApp was built in PHP and then people started chatting in status messages. Pivoting to be a chat system may not have been a massive pivot, but it was still a pivot.

Chat was added by running ejabberd[1], and the rest of the services (registration, status, sync) remained in PHP for some time, MMS was added as a PHP service. Services migrated to Erlang over time.

[1] Need chat? Run ejabberd is a gateway drug to running Erlang. Happened lots of places, but some then rewrite chat in another language later.