to be 'net busy' is to not be busy at all, however as you are on the internet you can say you are busy, and unless you're on webcam at the same time noone will know! this is used often when you don't want to talk to a certain person, or when you don't want to reply at all.
Very useful for messengers such as: MSN, AOL, AIM etc.
However, saying that you are net busy is suicide as they'll know you aren't really busy.

When you eat dirt for the first time and leave your family to build a shack in the woods