WhatsApp's mass adoption stems in part from how easy it is to find a new contact on the messaging platform: Add someone's phone number, and WhatsApp instantly shows whether they're on the service, and often their profile picture and name, too.Repeat that same trick a few billion times with every possible phone number, it turns out, and the same feature can also serve as a convenient way to obtain the cell number of virtually every WhatsApp user on earth—along with, in many cases, profile photos ...