WhatsApp went down for its 2 billion users earlier today, but service has now been restored. The issues started at around 4PM ET and prevented WhatsApp users from sending or receiving messages on the service for nearly an hour. The WhatsApp application would still open, but a connecting loop was displayed while the service experienced issues.
In theory, being end-to-end encrypted basically means they don’t need servers, but they do to route the messages to the correct device. To use push notifications to send messages (or more likely trigger message fetching in the background) and store (encrypted) messages whilst the recipient is offline.
I wonder if the original WhatsApp team ever prototyped ditching servers entirely & just started having phones peer-to-peer send?