Interestingly, I never had this feeling. My reaction was always like: oh, cool, this guy's there, that gal is doing this thing. Or maybe "well, that's a good idea, I'll try it go/there one day."

Maybe because I was aware of what you are saying, that everyone shares their best experiences and moments, and even those in an inflated way and on top of this, FB will find those and make those posts more popular (because people will react to them more strongly). Though thinking about it, I never had this feeling when I've learned about this IRL while I knew some people actually can get envy (i.e. feel bad) when they hear others are doing good or have fun even if they were 100% OK with their situation before hearing that.

I suppose my response to these kind of posts falls somewhere in the middle. Looking at somebody's vacation pictures doesn't make me specifically jealous to them, but it adds to the generalized malaise of being at the tail end of the bell curve. Sure, intellectually you realize you're probably average, but I think it erodes away at your subconscious.