My schedule was completely full with kids, lots of work, and little down time for hobbies or relaxation. But I got into the groove and was reasonably content. Enter Facebook's feed with endless updates from friends on vacations, attending conferences, exotic work travel etc., and suddenly I start viewing my own lot in a shitty light. My rational assessment was that things were fine, but I just didn't feel very good about it. I quit FB cold turkey and within probably a month these feelings evaporated.

I don't think fragile is a good description. I think it's simply reacting. If the shared family photos you mentioned started having a lot of stuff that you didn't like, you'd probably choose to avoid it.

I don't think it's any particular, discrete thing like that in itself, but rather the accumulated impact of the social media heuristic described over a period of years.

For a more extreme example, look at LinkedIn. Judging by the posts, one can easily be led to believe that all of one's connections are super-successful but utterly brainwashed corporate droids. It's a warped and misleading portrayal of human reality (for most of them, anyway—there are always those few that really do drink the Kool Aid that much). But it's naturally going to lead a lot of people to the anxiety: should I be as invested in my work as these amped-up salespeople? Am I working hard enough to promote my professional brand?

Yes, people are generally fragile.