WhatsApp updated its iPhone app yesterday and also dropped the price from $0.99 to free in the App Store. WhatsApp changed its business model for the iPhone app to make it the same as in other platforms where the app is free for the first year and then users pay $0.99 each year to use the app.
The company stated the changes in a yesterday:
"First, we are updating our business model for new iPhone users going forward. As you know, we used to charge iPhone users a $.99 one time payment, while Android and other platforms had free service for the first year and paid $0.99 a year after that. From now on, we’ve simplified our business model so that all users on all platforms will enjoy their first year of WhatsApp service for free, and only pay $.99 per year after that. We feel that this model will allow us to become the communications service of the 21st century, and provide you the best way to stay in touch with your friends and family with getting in the way. The good news for all current iPhone users is that WhatsApp will be free of charge for the rest of your life."
If you downloaded the app prior to yesterday, then you don't have to worry about paying $0.99 each year, it will be free for the rest of your life.