The 11 points below are summarised out of a very rich buffer post by Courtney Seiteron on the psychology of facebook
I found the post via an email from @abc3d & @hautepop.
I highly recommend you go to the post by Courtney to get more of the detail behind the points.
1) When you get +'ve feedback on facebook your brain's reward centre kicks in
2) Browsing Facebook can evoke happy feelings like when you're totally absorbed in an activity
3) Liking something is like nodding in agreement to show support to what somebody is saying
4) We mostly like/share posts consistent with our views & doing so affirms/shares our own views
5) Sharing supports causes, entertains, keeps us in touch & gives others a better feel for who we are
6) Via instant message chats we can be virtually empathetic with another
7) With brands and retailers we often like simply to get something in return like a discount code
8) Personal messages are more satisfying to receivers than one click likes
9) Commenting can train facebook to give you more of the content you want
10) Over-sharing from others can be one of Facebook’s biggest annoyances for users
11) Yet for those sharing the more they share the less lonely they feel (and vica-versa)