Well, to give you an example, here in Argentina, almost everyone in my phone's contact list uses WhatsApp. Here, the fact that you pay per message is a big incentive. But there are other advantages, like instant delivery, being sure that the message was received (double-check), better feedback ('X is online' when using the app, 'Last seen at 09:22', X is writing...), media sharing, groups, etc. simple things that help a lot. Also, it works both on iPhone and Android...