I agree with not comparing ourselves to other people's virtual facades, but really our online selves are just an extension of how we present ourselves in person, which is also not the whole picture of our lives. In fact, I know many people better through Facebook than in person, where all I can get out of them is small talk, if that - many times, we're like ships passing in the night.
And for introverts, this medium helps us to express the deeper thoughts that are harder to articulate on the spot with everyone looking at us and waiting for an immediate answer. But I am talking about writing, not about posting pictures and blurbs that really are akin to the same "in real life" interactions of fixing up our appearances and engaging in superficial conversations.
The best of both worlds is cultivating those deeper one-to-one friendships and small groups, and that same sort of authenticity (sorry for that word, but it fits) carrying over into our expressions online.
Granted, not everyone likes to write, so they cannot be blamed for only posting quips and pictures, nor should they be accused of only showing themselves in a good light. Not everyone wants to be vulnerable in this place, but that doesn't mean they are being fake or that that they don't reveal their struggles to those they trust.
So really, it's our problem if other people's posts make us feel envious, left out, etc. I have felt this way at times, but it's always because I haven't been spending time with that person, so I feel disconnected from them. That's when I reach out. If it's ignored, then I stop looking at their posts as often, so that I'm not reminded of the rejection. Eventually, if there is no mutuality, I may even unfriend them, because what's the point of only being connected to someone online if they are not interested in actual friendship with me?
Well, it's possible that they still read what I post sometimes and are encouraged, helped, or somehow touched...so then I have to put aside thoughts about myself and trust God that He wants me to keep that connection, even if it feels totally one-sided. That's also my calling as a writer - to minister to others without looking for my own gain. Someone may benefit from what I share without necessarily wanting anything else - I can either feel used, ignored, or not worry about it, and trust that God is working all things for good for those who love him.