The web based email provider GMX allows to import the facebook friends as contacts using facebook API. Does it mean it is possible to get the email address of facebook friends after authentication? If there is such thing, is there any tutorial or helpful resource regarding that. Thank you for your help.
I know the feature you're talking about. As far as I can see, what they do is, they take your GMX password (which you have to enter), call some API at GMX, log in there programmatically, and compare the E-Mail addresses stored there against those registered with Facebook.
There's nothing really special to it, but they almost certainly have a technical cooperation with GMX to do this. It's probably not an option open to us small-time developers.
It would theoretically be possible, upon getting the GMX password from the user, to do an automatic login and scrape the address book data, but unless they offer some sort of API, this is close to impossible to do in practice.