At WhatsApp we leverage FreeBSD and Erlang to provide industry record uptime. The ability to scale linearly on commodity hardware has allowed WhatsApp to keep our serving costs low. These days we run anywhere between 2 million and 3 million concurrent TCP connections on a single FreeBSD server as we documented in our blog post.
WhatsApp is the leading global mobile messaging service. WhatsApp Messenger is a cross-platform mobile messaging app which allows you to exchange messages without having to pay for SMS and is available for Android, BlackBerry, iPhone, Windows Phone and Nokia! Because WhatsApp Messenger uses the same internet data plan that you use for email and web browsing, there is no cost to message and stay in touch with your friends.
– Jan Koum, CEO aka Sr. Tweet Manager, WhatsApp