That just made my day..
A simple and straightforward argument FOR simplicity and convenience..There may be a dozen apps better than facetime and people may prefer one over the other...But Facetime brings standardization..Its on your phone all the time..Your freinds, family know whether you have an iphone or not..You want to call someone, just go to the address book and it will give you a Phone call, Face time or Face time Audio option all in one place..No hastle in trying to figure out WHICH apps to download to cover all your freinds and family..Unless you are the WOLF of your circle and dictate to everyone in your circle about which appls to download

..Facetime is popular because its always there in nearly all IOS devices (almost) and it does not require downloading apps or turning it on and what not..All you have to do is go to the phone launcher and decide whether you want to make a phone call, audio (FT) or a video Facetime call !
There is a huge customer base in the US (and abroad) that have an Iphone, Ipad or Ipad Mini and/or have Mac's that run facetime..Thats a huge number and all unified through Facetime...You could download other apps if you consistantly wished to speak to your buddies who ran android or windows..In my case i only use facetime to chat to my immediately family and my closest freinds all of whom have atleast one iOS device. I do use skype however through my mac and love it!