Could it be because time spent online is filled with upward/downward comparison ("I'm worse/better than people higher/lower on the social ladder", respectively), sensationalized apocalyptic news and people engaging in dishonest and malicious behavior a la 4chan?
This is where my money goes.
Unlimited access to information is a good thing when one is able to self-filter what they should and shouldn't be adding to their worldview. If we took a trip over to WorldStarHipHop or LiveLeak and spent hours, days, weeks consuming nothing but that kind of content, one would have a pretty bleak outlook on reality.
In my case, your first example of 'upward/downward comparison' is something that has been driving me nuts since I was old enough to get online without adult supervision (7-8 maybe). Disengaging from social media has been important to keeping some grasp of my sanity as a young adult, because it's right in that sweet spot of late-teens, early twenties where socio-economic disparity becomes most apparent. I always knew my high school was a wealthy one, but only as a young adult do I realize HOW wealthy. Example, one family bought their daughter a house in the city where she went to college, because they figured out it was cheaper in the long run to buy a house, fix it up, have her pay like $200 a month 'rent' and sell it when she was done with school, rather than pay for dorms/apartments. Tuition aside, they MADE money from her college living expenses.