Update (2014-02-20): Nathan makes a good point in the comments and I was also reminded of my post from about a year ago in which I considered the implications of just such an acquisition.
I was just reading the Facebook press release about its $16bn WhatsApp acquisition and noticed this quote by WhatsApp’s co-founder and CEO, Jan Koum:
WhatsApp’s extremely high user engagement and rapid growth are driven by the simple, powerful and instantaneous messaging capabilities we provide. We’re excited and honored to partner with Mark and Facebook as we continue to bring our product to more people around the world.
Also, bringing 450 million people into Facebook’s broader community does’t hurt either and the $16bn price tag gives you a sense of the value WhatsApp’s 450 million users has to Facebook.