How to link an email account to Facebook in Spring Social?

I have been some time trying to figure out how to solve this. Let me explain in detail to you.

I'm developing a web app in Spring MVC and Spring Social. By now, users can sigin and signup with Facebook, Twitter and with user/password. Even those users signedin with Facebook/Twitter are able to share.

Now, the webapp owner wants that any user signedup with user/password, he/she can link his/her active account with his/her Facebook profile. This way, those users can post on his/her Facebook wall.

Any of you have an idea about how to accomplish this?

By now, I have tried to get FB token through Graph API http link, but URL shows FB secret property.

Any ideas?