BuddyPress.org

I simply use the facebook connect. and have disabled any other form of registration or login.
and i only get 1 user unless they have multiple facebook accounts and i get the benefit of avoiding the need for any other authentication

since facebook spend millions confirming its users i get to use that development for free.

I do know however that 99.9999 % of my users will be facebook users

So when i look at yoru delema i wonder why you want to offer all those ways and in turn CREATE your own problem