The term internet addiction has been around for a while. I have always found this term offensive. When I was walking two hours to surf fifteen minutes on the internet, and when I was virtually plugged in all day - booting the computer on waking up, shutting down before sleeping - sometimes not even that if I had a download on. Neither qualified as internet addiction to my mind. The internet was a useful thing, I was using it as I needed to.
As a person who has always been somewhat a loner, and then going through depression, my social contact in the real world was almost zero. The internet kept me sane. At other times, I managed my websites and blogs. Still other times, I spent time on the net for pleasure. The internet is a tool, and how we use it determines if it is functional or not.
As entities, functions, information get increasingly virtualized, it is a fallacy to consider the amount of time spent on the net as a measure of addiction. From reading your morning news, to connecting with professional and personal contacts, running a business, writing, researching, socializing, entertainment... you name it, it is available on the net. I have the computer running all day. Sometimes I'll quickly check on some information, other times, I'll send messages, still other times have a conversation, watch videos, attend a virtual conference.... As the number of possibilities on the net goes up, so do the reasons people use it.
Another side of the net that I see is the opportunity for like minded people to connect. In our daily lives, our social circles are largely limited by who is available, and who among them we like. Meeting requires taking time out and physically relocating, and thus limits contact naturally. Online, most of these restrictions melt away. Day or night, someone is usually around. I think the social world online fills a very real need for interpersonal contact. As lives get busier and people find less and less opportunities to meet like minded people, I think many of us are deprived of company. This need is filled by social networks. Many will remain connected with social networks through their phones - virtually online all the time.
There are other aspects. Interests from obscure subjects to common "vices" like porn, games, etc. I don't see either porn or gaming or anything a person can do without harm to another as evil, so I suppose this takes out a large part of the "evil factor" for me. And there is the dark side - like the real world. Cons, exploitation, child porn, piracy, etc. Pretty much like the real world.
To most people used to remaining connected online, it is as much a regular element of life as the desk that computer sits on.
Whether this can be called an addiction with any logic remains to be seen. Sure, some people have withdrawal symptoms if they cannot get online for long. But then, we have those about mobile phones too. Anything you come to depend on going missing will unsettle you. This doesn't mean that depending on that thing is bad.
I can understand that doctors and other professionals who make such diagnoses often have busy practices with real people, and neither experience the need for constant company, nor do they have the time to utilize it. This may make the dependence on the internet seem unhealthy to them, like the dependence on having own transport on hand seems unhealthy to someone used to using public transport all their lives. The person traveling by public transport may not appreciate the comfort offered by familiar, reliable transport fully under their own control to someone who spends a lot of time traveling. What we don't understand and don't need seems superfluous to us. Depending on something unnecessary may seem like a bad thing. Does not necessarily mean that what is not useful for one is not useful for all.
I have met different kinds of people who spend a lot of time on the internet. Most have a purpose. Even if that purpose is socializing or entertainment. It seems strange to me to render that meaningless by calling it an addiction - as though it is not utility, but compulsion driving the behaviour.
Sure, there may be cases where this gets dysfunctional, but that is no more or less than what happens in daily life. There are people who while their days away watching TV, reading, anything. I think more useful than creating labels like internet addiction, it would help all to research virtual behaviour, and possibly start a new branch of psychology that can actually help understand what is happening, and thus address dysfunction better.