When using FaceTime on a Wi-Fi connection your data plan is not used. Similarly, no cellular calling minutes are used during the video portion of a FaceTime call. FaceTime calling can be used over a cellular data connection with an iPhone.
But one often overlooked aspect of FaceTime is that of its audio function. This works in much the same way as video, but can be used to bolster the call minutes on your mobile phone tariff, and even turn your iPod touch into a mini iPhone.