I completely get what you are saying! I left facebook altogether and part of the reason for that was because of feeling exactly what you are feeling. I, too, know better than to think that everything is peachy for those people but you can't help but feel like there is more good going for them than bad.

And it's not just an age thing. I'm 25 and feel the same exact way as you do seeing people my age being accomplished. But then again, I'm on a quest to finding my own path and what makes them happy might not make me happy as well. For example, sure, they might have a great social life but maybe what they look for in other people is suffice for them and not for me, if that makes sense. So instead of being envious that they have friends, I question how meaningful is their relationship with the people they know. Are they seemingly partying it up to exaggerate how great their life is or are they really content with their lives being at that place? But that could be my bitterness talking.