This is it. The social media depression effect is that you're always comparing yourself to the top of the bell curve rather than the middle.

Suppose you have 52 friends who all go on vacation one week out of the year. Every week you will compare yourself to whichever one who is posting their fabulous vacation photos. This doesn't mean you're having a worse life or missing out, you're completely average. You just only notice the 51 comparisons in that direction and don't notice the 51 comparisons during your own vacation week.