Mental health and time spent online

I just made a casual comment to a friend of mine that is going through a rough time about the amount of time I see him on his Smartphone when I am around him. Several studies have popped up in my feeds in social networking resources that I use establishing a link between poor mental health and large amounts of time spent online. A quick search of related articles include an article published in the UK claiming that the government warns a causal relationship between time spent online and mental illness in children, another article warning that teens spend more than 2 hours per day on social networking sites are more likely to present signs of poor mental health and finally another reporting on a study in Sweden which claims a 'central link' between time spent online and sleeping disorders and depression.

Although my limited research seemed to indicate that most studies and articles published on this topic focused largely on children and adolescents, there is also a significant amount of research that examines an adult population.  However, I think when reading on this topic we need to pay attention to the difference between correlational and causal relationships.  While the majority of the articles discuss a "link" between time spent online and mental health, there was little discussion of how exactly, these two issues interact.  What is more likely, that high quantities of online use cause mental illness or that mental illness causes increased use of the internet?  I think the other option would be that the relationship is cyclical, that one of these characteristics feeds into the other and so forth.  Also, it would be interesting to consider how strong the relationship is.  In other words, clearly not everyone that spends high amounts of time online suffers from mental illness or vice versa.  However, it seems like an interesting concept to examine further.