An interesting study as summarized on a PT blog:
According to a 2014 study published in Computers in Human Behavior, most people aren't using social media to be social. Only about 9 percent of Facebook's users' activities involve communicating with others.
Instead, most users consume random pieces of content. And researchers found that passively consuming information isn't fulfilling or satisfying.
Study participants experienced a sharp decline in their moods after scrolling through Facebook. Interestingly, they didn't experience the same emotional decline when they surfed the internet. The toll on mental health was unique to Facebook.
Through a series of studies, researchers concluded that by the time people log out of Facebook, they feel like they've wasted their time. Their remorse over being unproductive causes them to feel sad.
Has this finding been corroborated by other studies on "social networking" sites? I put the term in quotes because if this study is confirmed, then Facebook seems involve little actual social networking. So perhaps a new term is needed... "fake social networking"? (heh.)
The same PT blog cites a more recent (2016) paper (and actually gives a link to this one, unlike the 2014 one) which proposed a different explanation for the Facebook-induced sadness, namely friends envy, especially for users that have a more consumer/passive participation. I'm actually less interested in untangling an underlying factor (in this Skeptics question) than confirming that a sadness-inducing effect is common for Facebook users.