I rarely call people by nicknames. I have yet to break this habit with you, Sherlock Lupin. I only make nicknames (or shorten) when the name is atrociously long in most cases.
Let's see:
The only people I call people differently from their screen name:
IdentityUnknown: Jean
Parkur: Parks
Misaki-chan: Misaki
Kid the Phantom Thief: KtPT
Kaitou Kid Legendary Thief: (or KtPT, if I'm a bit out of it :V)
Koizumi Akako: Chelsea
Aeyra: Aya (?)
Chekhov MacGuffin: Chek
PhoenixTears: PT
I broke the internet box.