Why is Facebook so popular? My theory is that it makes people happy.
When you post on Facebook, only your friends can comment. Friends are more inclined to write kinder and gentler comments than critical or hostile ones. To fix the idea, imagine a fictitious Facebook user Ma. Ma's enemies are unlikely to become his Facebook friends, and as a result, cannot make critical or hostile comments on Ma's posts. Consequently, Ma reads the favorable comments from his friends and is a very happy man. The happier Ma is, the more he likes Facebook. This is known as preaching to the choir.
Not only are Facebook people protected from enemies, their friends are only allowed to like a post, never dislike it.