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.