Hi just had a look for ya and i think this this is how u do it.
1.Go to ur facebook page
2.At the address bar on the top of ur profile page, copy and past the "url address"
3.Then come back Go to User cp
4.Click on edit details
5.Scroll down right to the bottom
6.Then u will see a box for "url for facebook. our site
7.Paste the "url address u have taken from the address bar of ur Facebook
8. Make sure u save it
Think that should then update ur profile so the Facebook sign then comes up on ur profile, i have not done on my profile but i think this is the way, hope that works xx