Cost per user is moot when compared against companies which inherently have different users, across different types of devices. Take our latest spot of head-scratchery ala WhatsApp, it's easy to do math and say: "oh right they paid about 40$ a user." Did they really?
WhatsApp has a huge set of "new" users on new and old devices. So, how again is a WhatsApp user on some random (non-smart) phone in India, as "valuable" to Facebook as one sitting in the Valley with their iWhatever.
Why is this surprising? This happens with every single item of news worth getting up in arms about. You are just seeing the receding area under the bell curve of social interest...