All users have an email address associated with their account (for registration and login). Like zzleeper said though, some are probably hidden. Also, you may

All users have an email address associated with their account (for registration and login). Like zzleeper said though, some are probably hidden. Also, you may have gotten multiple addresses for a single contact. For example, I created my Facebook account when I was in high school using my Gmail account, but when I started college, I had to add my college address to join my college network. So my Facebook account contains both my personal and college email addresses.