Some users' user name contains space. My question is:
To @ those users whose user names consist of two part, which one of the forms in the title I should use?
My name has a space in it, and if someone I don't know is being polite they should use a space. Similarly, I would use a space for any other user's name if it has a space, and not abbreviate it in any way, unless I have interacted with them a few times and think it is now appropriate. This is the same kind of common courtesy I would use in email or any other form of written communication.