It is well known amongst certain *cough* communities that the electro-magnetic forces produced by modern technology have a harmful effect on invisible life energies (although strangely they have lots of active shit on teh interwebs). Of course, these problems can be avoided through the strategic placement of expensive crystals and the wearing of a tinfoil hat.
Mobile phone masts cause a tubercular miasma that is injurious to mental health but this can be remedied by a 30C dilution of Nux Vomica.
Actually, I'm having mental health issues at the moment and in some ways social networking is a life-line in that it lets me interact with people when otherwise I would be totally isolated. Oh, sure Twatter and Facewank, etc, is a bit pointless, but I suspect that without the support I get from some of my internet contacts (I hesitate to call them "friends" because the Facewank thang) I would be in a much worse situation.
In my case, the support I get helps me re-build my confidence and is enabling me to learn to do real world social stuff again.
I do not doubt that there is a correlation between people who are extensive users of social networking and increased morbidity and mortality. As per other comments - correlation <> causation. But, if you are looking at the overall picture, you lose some of the detail. There are groups that probably have had a reduction in morbidity and mortality as a result of what the internet can offer. I know old folks whose life has been considerably enhanced by Skype.
But it's about enabling stuff rather than it being an end in itself.