I call online people by their screen names and nicks, or shortened versions of them. For example, I call Kawaii Ryűkishi "Kishi", Loony BoB "BoB", Psychotic "Psy" etc, and many call me Seph (or Sephy). As for IRL, I usually go by real names or nicknames that people have, unless that person's screen name is somehow related to his or her real name. As another example, if someone's name is Bobby Corwen and his nick is Boba or Corwie, I'll probably call him by his nick. There's a girl who calls me Sephy even IRL, though she's the only one who does so, so I'm not used to being called that except online.