"Friends"
I’ve got someone I would consider a friend (at least an online friend, if nothing else) who just outright refused to tell me his last name. I’m sorry, but that’s just lame. How can I consider you a friend if you don’t even want to give me your last name?
I guess for me it’s a trust issue. If you won’t trust me with your last name, why should I bother telling you anything about me?