Society considers social media to be the norm now, so for someone to actively avoid participating in it, they either are so successful and busy that they don't care for social media, or they don't have anything to post/brag about on it.
I think in your argument above you can replace "social media" with any number of things, like working out for example.
Impression management is a phenomenon that has increasingly taken hold of people's lives. One may think that if he/she doesn't post something on social media, then he/she isn't as "cool" or "successful" as his/her friends that do post frequently. People seek to highlight the best versions of themselves, or rather the person they
want to be sometimes more than the person they
are (Check out Impression Management sometime on Google, interesting stuff).
On the other hand, you could make the assumption that those who do post frequently are compensating for something. Someone may post pictures from every single one of their workouts, for example, leading me to wonder: "where do you find the time to commit yourself to work, or familial relationships?"
Again, same goes for any number of activities that society deems normal.
Posting about something doesn't make it any more "real" for me. I have friends that appear to "live" on Snapchat, Instagram, and/or Facebook. Some of these people I haven't seen in two+ years, yet I feel as if I know every single detail of their lives. I'm old school, I don't hate technology or social media but I do hate the "must post" aspect of social media.
I applaud the resolution you're considering. I would start slow, depending on how you use social media that is. For example, I deleted Snapchat about a month ago after using it pretty frequently daily. The longer I go without it, the less and less I feel the "need" for it. I think we can become addicted to social media just as someone can become addicted to something harmful to their bodies, like substance or alcohol abuse. I'm sure years from now, if not today, there'll be studies that describe the addiction to social media being potentially as mentally-harmful as substances.