People see curated snippets of others' lives and think they represent this accurate reality in which every person but you is in the leisure class, world hopping, displaying opulent luxury at the height of taste, and never even having to work! So depression goes through the roof, and keeping up with the Joneses becomes keeping up with the Musks or something like that.
The young people especially suffer terrible effects on their self esteem because of these things, and adults don't faire much better. I've heard so many interviews with social scientists who specialize in this, and the analysis makes you just weep for what's lost.
On the other hand, social media does also provide places where some people can find like minded individuals they couldn't otherwise, and that's a miracle for those people, especially those who are marginalized, targeted, ostracized, etc.