Believe it or not, Facebook is a handy tool. I used facebook to get a password for a login once, it was the guys wifes first name so I facebook'd his email and saw who he was married to.
You might be able to do the same thing, usually you're able to search through their friends they might have added their family, find their mother and pretend to be someone who went to highschool with them. If her name is something like Sarah Connor but you know that's not her maiden name, be like..."Is this Sarah White? I think I recognize you from somewhere but I can't put my finger on it."
She'll probably respond with her maiden name. You just have to have some basic social engineering skills.
It's so basic yet so powerful.