I read today psychologists have noted a phenomenon called "Facebook Depression." Essentially, viewing friends' profiles, with their smiling photos, and their posts about their activities gives one a skewed view of the world. One compares one's own life not to the actual lives of others, but to their lives as posted on Facebook. Suddenly, one feels like a loser, with a relatively uninteresting and unenjoyable life.
I've experienced this, somewhat with Facebook, and a whole lot with Tumblr. All the beautiful photos of the beautiful people doing beautiful things on Tumblr tends to depress me after a while. I need a Tumblr diet. Or a total internet diet.