What's notable about those numbers is that WhatsApp is getting close to the level of daily global SMS traffic, which is generally estimated to be between 19 and 20 billion messages per day. This has been achieved by a company with around 25 engineers and 25 support staff.
Taken as a whole OTT (over the top) messaging services like WhatsApp, WeChat, and BBM are thought to have exceeded SMS traffic for the first time in 2012, but the fact that WhatsApp is getting close, just one year later, to doing this on its own shows the remarkable growth rates of the service. There are still many more SMS users than OTT messaging users, but the latter send a much higher average number of messages a day (32.6 versus 5 according to one estimate by Informa).
At DLD14 Koum also reiterated the "no ads, no games, no gimmicks" philosophy of WhatsApps, noting that he believes the personal nature of messaging make this a key point in favour of WhatsApp. He also explained that the company's current focus is on creating an easy to use, fast, and reliable messaging experience, with the subscription based revenue only a secondary consideration.
Here's a video of Koum's appearance at the conference. The discussion, moderated by Wired UK's David Rowan, also includes some background on the company and Koum personally:
The WhatsApp app for Windows Phone can be downloaded for free from the Windows Phone Store.