title: What is "U" means in "gender" value?
body: <p>I found "Gender" value in page_fans_gender_age (/insights/page_fans_gender_age/lifetime) state as M, F and U. What is U stand for? </p>
  
  <p>This is could be undefined or Unknown. Just looked on registration page there has only Male and Female and it is mandatory field. If this for unknown, how FB define this?</p>