I don't know about others, but anything that requires me to log in with Facebook ends up stopping me from using it because I don't want all my contacts to see that I signed up for it.
Easy for you guys to implement, yes. "Viable"? I think not.
If I can't see why I might want to join without joining, I'm not going to join, plain and simple.
* I don't know where these stats are, or what the figures actually are. People just keep telling me this over and over again.