Facebook use predicts declines in affect and life satisfaction over time.

Because most people only post the best aspects of themselves, it makes those reading those posts feel bad about themselves since they never see the other side of the lives of their friends. They only see the weddings, the baby births, the job promotions, the parties and none of the day to day crap everyone has to deal with.

And it is a constant invitation to compare yourself with a biased picture of how other people claim to be doing.

Because of this, social media becomes a toxic cesspool that breeds status anxiety.