i have seen the word " şşşşiiitttt" used a lot on facebook etc... just wondering what it means out of curiosity... 

hey, look here, pay attention to what i am saying...

and be quiet.

http://imageshack.us/photo/my-images/250/foto176ha2.jpg/sr=1

if the ş (sh) sound pronounced long, it means be quiet.
if the ş sound very short, it means listen.

in online world over internet, no one can hear your typing, you are not talking at all, so it always means: listen to what i am saying, pay attention...