FaceTime calls use data. One advantage is that wireless calling plan minutes are not consumed. On the downside, when using FaceTime over a cellular data connection (not Wi-Fi) the data plan is used.
With most data plans now capped at monthly allotments, keeping track of data usage can be important for wireless subscribers. The amount of data used by a FaceTime video call or FaceTime audio call can be seen in two different ways on an iOS device: