WhatsApp is free for the first year. Once people are hooked, that's when they bang you with the annual fee, which is 99 cents. That's right. People with an unlimited data plan can message until their 4G radios burn out and it still costs less than a dollar a year (although international charges might apply in some cases). The only other thing you need to know is where to download WhatsApp Messenger. There's also a Web version.