Group message will probably be the "killer feature" in iOS 7.
Until then, use WhatsApp - works perfect and supports iOS, Anroid, WP, BB, Symbian. The problem is that you can't switch from iMessage to SMS easily, because you can only send SMS to one person.
Isn't Facetime P2P? I think it would require a server-based solution to get group Facetime to work properly, especially over 3G. Besides that, it doesn't make much sense on the small screen of the iPhone. Not even Skype supports group video chats on iOS. Would definitely be a nice feature on the iPad and on OS X, especially to save the money for the Skype premium account.
But in the end, proprietary solutions are not the way to go for communication, unless you select your friends by their mobile operating system.