Actually I was speaking about addiction more generally, but yes actually there are lots of other things to be addicted to. Sex, drugs and alcohol immediately come to mind. The Internet, while quite interesting and addictive in its own right, does not provide quite the same physical sensations. And often the choice isn't between "nothing in particular" and your vice of choice. Part of what makes an addiction harmful is that when faced with a choice between healthy relationships and your addictive vice, you choose the vice (and ultimately isolation, which works to increase the grip of your addiction).