How to know someone e-mail address

1. Log in to your Facebook account
2. Send your target a friend request (If you dont have him yet)
3. When he accepts your friend request go to http://www.yahoo.com
4. Then click on "Sign In" (its in the right of the site almost in the top)
5. Click on "Sig In with Facebook"
6. It will redirect you to the Yahoo facebook page/app, click on "allow acces"
7. Go to your Yahoo profile (you are already logged in)
8. Go to your contacts and select "import contacts" there will be appear a option to import contacts from facebook, select it.
9. When its done, you will see their e-mails below their facebook usernames.