For reference here is a talk they did about how they scaled WhatsApp and the problems they faced: http://www.infoq.com/presentations/whatsapp-scalability http:

For reference here is a talk they did about how they scaled WhatsApp and the problems they faced: http://www.infoq.com/presentations/whatsapp-scalability http://www.infoq.com/presentations/whatsapp-scalability