Researchers have found that there is a link between excessive internet use and depression.
It was reported by this Fox News article, that the more time one spends on the internet, the more likely it makes you depressed; the less likely you would be happy.
The depression is also linked to addiction. It was found that the average depression score of the hardcore internet users who spent many hours on the internet each day was more than five times higher than that of non-addicted users.
Researchers are not categorical about the cause effect relationship here however; whether it was the internet that was causing the problems or whether it was that people who already had mental issues were drawn to the internet. There may be some basis in the perception that over-engaging in web sites that serve to replace normal social function might be linked to psychological disorders.