The headline number is $16b, for WhatsApp being acquired by Facebook.

What it does
"WhatsApp Messenger is a cross-platform mobile messaging app which allows you to exchange messages without having to pay for SMS.

WhatsApp Messenger is available for iPhone, BlackBerry, Android and Nokia and yes, those phones can all message each other!

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."

Sequoia Capital invested $8m in WhatsApp (founded 2009) in April 2011.
Dan Primack, Fortune, says the total investment was $60m, and that Sequoia stand to make around $3.4b on the deal.

Infrastructure : 450m active users. 32 engineers " a reliable, low-latency service that processes 50 billion messages every day across seven platforms using Erlang" Lots more detail collected from a Rick Reed talk in the High Scaleability blog post.

References

Sequoia Capital http://sequoiacapital.tumblr.com/post/77211282835/four-numbers-that-expl...
Wired UK background http://www.wired.co.uk/news/archive/2014-02/19/whatsapp-exclusive
Fortune numbers http://finance.fortune.cnn.com/2014/02/19/facebook-whatsapp-the-other-nu...
HighScaleability summarises Rick Reed 2012 video http://highscalability.com/blog/2014/2/26/the-whatsapp-architecture-face...
Rick Reed's slides https://github.com/reedr/reedr/blob/master/slides/efsf2012-whatsapp-scal...