Facebook recently updated the iOS app to version 5.5, which included the same free calling feature its Messenger app has had for a while now. As with the Messenger version, you're able to place or receive a call over both a Wi-Fi or 3G/LTE connection from your iPhone.
The option to place or receive a call is available only on the iPhone, not on the iPad or Web site.
As you can see in the video above, placing a call is simple:
Launch the Facebook app and view your Messenger contact list.
Open a conversation with a contact, then tap in the "i."
If the contact is using either the Messenger for iOS app, or the most recent Facebook for iOS app, the "Free Call" button will be highlighted. Tap on the button to make a call.