Facebook Email Extended Permissions by Default

I use the Javascript Facebook API to request advanced permissions for email. It works, and the user can choose between a real and a proxy server.

My application uses the proxy email address by default. I thought it was the default method for privacy, but now my manager saw this site: http://www.tripadvisor.com/ , in which the default email address was selected instead of the proxy server and asked us to implement same.

What determines the default choice? facebook settings? I did not find anything official in this.

This is the code I'm using.

FB.login(function(response) {
      // do some stuff
}, { perms: 'email' });

Thank!