It might also be the passive nature of the two activities.... browsing facebook and watching TV are very passive activities. You do not use your creativity or skills to do them; happy people tend to do more creative and constructive things.

A lot depends on the way one uses it. Personally, I (and many of my friends) use it the same way I use HN - I post interesting stuff and engange in long, asynchronous discussions that sometimes span many days.

It's the medium, not the message. I would suggest that it's excessively stimulating and this causes maladaptive neurochemical changes. The same applies to computers.

Of course, no one seems to understand the precautionary principle. If something is potentially implicated in health issues, the rational reaction is to exercise caution. Instead, it's full-steam-ahead, and before we know better it's too late.