My wife spends twelve hours a day on the internet. Can she actually become addicted?

Possibly. These problems are termed "process addictions" and it is poosible to become addicted to any activity. Are necessary activities and social interactions suffering? Is she aware? Is she unable to stop? Than she is addicted.
Internet Addiction. It is not an addiction in the same sense as an addiction to a physical drug but people do become obsessed with the relationships on the internet and some can't seem to give up their imagined world. Facebook and other media sites such as chat rooms offer imagined relationships that can be more attractive than reality. Talk openly with your wife and tell her how you miss her presence. Best wishes.