So, you put good news on Facebook because you're in the process of presenting yourself and you want that presentation to be positive. But reading all of these positive presentations makes you depressed - you feel happier if you think your life is better than everyone else's. So the end result is that social media is making people unhappy because they don't like to think that everyone else is having a good time.

People are fucking hopeless.

Good News Spreads Faster on Social Media - NYTimes.com