Which I find funny as hell that people don't just use sms, at least in the US most cell plans are now unlimited text (not including pictures). I never quite got the point of whatsapp or other text message alternates in the US... especially when they (many) don't have a website that can be used from the desktop.
Three reasons that spring to mind for me are encryption, rich content at no additional cost (videos+images) and the ability to know whether a message was actually sent/received
