It's linked to your forum name, not your actual email address, or your home address. For instance, mine is listed kamui@kamui. Guess what my email address ISN'T. Either of those names. Don't worry, you're safe.
If you can see his login name, it's his own damn fault. When you create the account, you are prompted to enter your username and your forum handle. Do you really think LUE_The6BillionKgMan is my login name?
If you or someone you know didn't bother reading when he was creating his account (most people on these forums can't read, don't feel bad), you can contact customer support and they'll be more than happy to change your forum name.
I can see a guys log in name on my friends list. Are you nuts? Have you ever heard of hackers?
This is a stupid idea to let anyone see my log in name.
I had my account hacked in WoW.
This is just asking for trouble.
Your login and @global are both Heartshot? Hmm... Yeah mine are different. You'll never guess what my actual login name is. No, it's not Metallurgist. It's your own fault for setting them up as being identical.