I suspect it's some magical data compression technique, because my overseas video calls usually suffer from serious stuttering on Skype, but on Facetime it's smooth as silk.
Either that, or better bandwidth because less people use Facetime.
Better bandwidth is not a function of number of users, but of what you want to spend per user.
I don't have personal experience comparing the qualities of Skype vs FaceTime, but I guess Apple is willing to spend more per user. That makes sense: FaceTime sells hardware, and that brings in money, while Skype seems more like a land grab, especially with this 'group calls are free, too' change. It could also be defensive, with Microsoft thinking they have to offer this to prevent would-be Office 365 users from migrating to Google docs.
