>>Risk factors are lonliness, low self-esteem, unattractive appearance, unemployed/part-timers, and a pattern of failing relationships.<<
Much of this sounds on target to me... though I might change the "unattractive appearance" to something like "social anxiety." Although some internet "addicts" may or may not be physically unattractive, I think they often lack - or FEEL they lack - f2f social skills.
Some people obsessed with the internet are obsessed with cybersex and online romances. So I think there is a thirst for affection, sexual outlet, and/or simply "feeling special" in the eyes of someone.
I'm reluctant to make broad generalizations about people who spend a lot of (too much?) time online. It's a very heterogeneous group of people who have many different motives and goals. For those who go through a "phase" of internet preoccupation, the motives can be healthy. Did I mention this article that I wrote about online and computer addiction? The important part, me thinks, is the "integration" principle that I discuss near the end:
http://www1.rider.edu/~suler/psycyber/cybaddict.html