LadyZero, I just now read your post and it occurred to me that this emotion has been part of my life the entire time I have a facebook account! Which is probably 2 years. For me, what makes it worse is that my husband is very social (I'm not) he had a long full life before he met me, he has a good job (I don't), and gets to attend all kinds of events I don't. Therefore, he himself is one of those people whose FB posts I don't necessarily want to see!
But the main issue is others. I literally cringe at the pics of those with big families who get along (our family is very small and barely gets along.) I only have one sibling who has refused to have anything to do with me for sixteen years and my husband's only brother is dead. Any pic of a woman friend with her sister (or sisters!) many of them have several is a stab in my heart. Our family has also moved around alot due to my husband's job, and for that and other reasons I personally never had had a large group of friends. Around here, almost off the women are constantly posting pics of their girlfriend groups, things they do with them, etc.
My envy is more for the many friends I don't have and the sibling relationships I don't have than for material envy. Material things are often "shams" in that some people buy with money they don't have and show off. But it's hard to "show off" friends or family members you don't have if that makes sense.
Yes, I truly have fb "depression" and wish I didn't.