I know I don't mind if people use my real name - it is in my signature and all - in any posts (Christina!)

I know of a few of my friends that I know their real name, but they'd rather I didn't tell everyone what it is - so when I respond to a post, I use their username. There are other people who don't mind me responding to posts using their real name. For some other people, I'm just really bad with names so I'll tend to use their username in posts, until my brain allows me to memorize another persons name.