When you sign up on Facebook, you have to enter an email address and that email address becomes your primary email address on Facebook.
In a recent disclosure by a Security researcher, Stephen Sclafani - The Social Networking site Facebook was vulnerable to disclosure of primary email address of any Facebook user to hackers and spammers.

In a recent disclosure by a Security researcher, Stephen Sclafani - The Social Networking site Facebook was vulnerable to disclosure of primary email address of any Facebook user to hackers and spammers.