Came across this interesting article on Facebook authentication. "Shoppers authorize sharing of their Facebook data with online retailers 56% of the time"..
For the 42 applications, we found that the Facebook Permissions authorization rate was 56% on average. The median rate was 58%, and the first and third quartiles were 50% and 66% respectively. Users authorized the use of their social data by clicking “allow” on the permissions request dialog.
To me this suggests that people need an alternative method of registering for the remaining 44%.