Currently this feature is available for the iOS version of Facebook’ mobile app and only within the US. There is no need to update the app to get this feature. To initiate a call, you have to go through the slightly convoluted process of going to the person’s account, starting a char with them, then clicking the ‘i’ button in the top right corner and then clicking on the ‘Free call’ option.
The voice quality over data is good and on par with other services such as Skype, etc. The app shows a push notification when you receive a call.