All terrible jokes aside ( Perceval ;) ) this is a very real thing. I used to be addicted to World of Warcraft. The game is so notorious for it that a lot of people in the internet's gaming community call it World of Warsmack. I ate up my life for about two years. But it isn't just video games, simple internet use can become compulsive, again speaking from personal experience. Even now I can get lost in the net, look at the clock, and realize I've been surfing for hours, burning time that could be used for better things.
However, I think labelling it as a mental illness is a bit much.