I don't think instant gratification is all there's to it and I don't know why every discussion of Instagram needs to describe its users as narcissists.

I regularly use Instagram as a photo archive to look back on travels, places I've lived, things that happened last month, etc.

And the general effect from my timeline is that I keep being reminded of the health and happiness of my friends, which has nothing to do with narcissism or instant gratification, really.