Concepts are mental activities that identify disparate concretes by ignoring some aspects of their differences. As such, we can unify pretty much anything under a concept. The question is never "is this abstract thing really real for reals?" It is: "Does this way of grouping things give us any useful way of dealing with the underlying reality?"
Debates about whether the constellation of phenomena that get united under a named concept constitute a "real thing" are almost always undertaken in more-or-less willful ignorance of this fact. The very way of posing the question--"Is this real?"--obscures the actually interesting and important question: "Is this useful?"
Addiction is a fairly well-defined and useful category of pathology. People who are addicted to something have similar symptoms and issues, and treatment of addiction can be usefully studied across groups of addicts, so what is learned about people with alcohol addictions may be useful to people with heroin addictions and so on (or not: simply because two things are grouped under the same concept for a given purpose does not mean they are usefully identical for all purposes, as anyone who has handled a sail boat and a row boat--both members of the abstract category "boat"--knows.)
No concept has perfectly crisp edges, and on the borders there are plenty of cases that one person would count as "in" while another would count as "out". If enough such cases start accumulating and impede communication and thought, we create new interstitial concepts. In the case of "Internet addiction" the interesting question is, "Is this enough like other addictions to group it usefully under the came concept, or do we need something intermediate or possibly orthogonal to usefully categorize it?"