Turns out I was right - much of the Instagram and SnapChat usage is coming out of the pool of attention that previously belonged to Facebook - but the cycle time seems to be ~10 years, and getting longer with each generation of social software. If the history of mass culture & mass consumerism repeats itself, eventually the cycle time will be greater than a generation - forms of social interaction for the baby boomers stayed roughly constant for their whole lifetime. At that point it's meaningless to talk about shifts to the next big thing, because we'll all be dead before they happen.