You are right. There is a spectrum between both. Since December, it has been harder than usual (nothing special, just too much stuff going on) and I use the net as an escape, but I think that I'm in this gray area where one must be alert. It is not addiction yet, but I should watch myself with a rather severe look and see if I can change the trend.
As an example, I play casual computer games to take breaks. So popping open my current favorite can be a habit. But I've had friends addicted to WoW, such that they were spending enough time in it that it was making their lives worse. The more they played, the worse their personal problems got, causing them to seek more relief through WoW.