They don't use it probably because it's too damn hard to find. From the Mac, Facetime is another app, double-click in your dock or application folder. From the iphone, not nearly as simple. First you have to open the phone app then select a contact, then select Facetime.

Unlike Skype, you have no indication that this person has a facetime compatible phone. Just to get it to call the desktop is problematic because my phone has my phone number as the username and my PC has my email as the username so your never sure where a facetime call is going to end up, and that is if it picks up.