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.
