WhatsApp is a good contender: <a href="http:&#x2F;&#x2F;highscalability.com&#x2F;blog&#x2F;2014&#x2F;2&#x2F;26&#x2F;the-whatsapp-architecture-facebook-bought-for-19-billion.html" rel="nofollow">http:&#x2F;&#x2F;highscalability.com&#x2F;blog&#x2F;2014&#x2F;2&#x2F;26&#x2F;the-whatsapp-archi...</a>