How Common Is It?
A variety of studies has reported an occurrence of Internet addiction as high as 3% to 10% of the population, but these studies suffered from self-selection bias, because they were online surveys.
Two fresh community studies of youth informed about a much lower prevalence. A Korean study of over 1500 15- and 16-year-old girls and boys who answered to a questionnaire reported an Internet addiction prevalence of 1.6%. Correspondingly, a Norwegian study of over 3200 adolescents found that 1.98% of this population was addicted to the Internet.

In order to explore problematic Internet use in the adult general population in the United States, Dr. Aboujaoude and colleagues performed a random-digit-dial telephone survey of 2513 adults in 50 states.
Of the people who were interrogated, 56.3% responded to the survey. The respondents had an average age of 48 years.

From 4% to 14% of the survey respondents demonstrated sings of some aspects of problematic Internet use:
  4% informed they were absorbed with the Internet when they were offline. 
  6% had private relationships that suffered as a straight outcome of unsuitable Internet use.
  6% frequently went online to escape from depression or negative moods. 
  9% were mysterious and felt they had to conceal their Internet activities. 
  11% often stayed online for longer than they planned.
  14% had a very hard time staying offline 4 days in a row.

Determining problematic Internet use as meeting 3 criteria (preoccupation with the Internet when offline; failed attempts to restrain use or often staying online longer than planned; and interference with personal relationships produced by Internet use), the scientists discovered a occurrence of problematic Internet use of 0.7%, which was the same for both men and women.

The scientific team concluded that potential markers of problematic Internet use seemed to be present in a sizable proportion of adults”. Additional studies are required to establish whether this is a separate pathological behavior or whether it is a symptom of other psychological pathologies.