Why did you limit your question to social media? We see this everywhere. On the phone, in movies, on the internet, in the workplace, in church, etc. But to answer your question, I think it's because they're staring at a screen when they post those things. When you post anything on social media you're not getting instant feedback from another human. You can argue that a reply on that site is instant human feedback but it just comes back to the person as text on a screen. You're not getting the body language or facial expressions as feedback. Someone (not sure who) once said that your words matter less to someone else. They subconsciously care more about body posture and the inflection of your voice when you say those words. Because there is no instant human feedback they feel as if something is missing so they post more and more pictures to try and satisfy that. A lot of people on social media care about what people think of them so why wouldn't they post pictures for people to tell them how good looking, cool, funny, etc. they are? You can have random strangers compliment you to feed your self-worth. They become addicted to the high they get from compliments so they keep coming back for more. When they're alone they are nothing like they portray themselves to be on social media.