I am on facebook (elizabeth renowden) to add it go into user cp tag at the top and click on edit your details - at the bottom there is an option to add your facebook it will say
"Facebook homepage url. If you do not have a vanity url be sure to take the "profile.php?id=XXXXX" where XXXXX is a number"
I got my details by copy pasting the address bar when I went into facebook. (had two windows open at the same time - one for facebook one for minimins).