Even though it's been around for a couple of years now, FaceTime has really struggled to gain massive user adoption. Mac and iOS owners just aren't using it as much as you think they would for a free, no-hassle video-calling service.
One of the main reasons for its stumbles is the fact that iOS devices have not been able to make FaceTime calls outside of Wi-Fi networks (unless you're jailbroken obviously). And it doesn't look like that's going to change with the new iPad...