Would be interesting to see the demographics on those users. My understanding is that if you're under 25, you use SnapChat, and if you're over 25, you can't figure out how the hell to use SnapChat. Instagram Stories may've exploded into the demographic that doesn't use SnapChat. This is possibly supported by the comments here about Instagram's UI being a lot nicer; most of us on HN are in the demographic above SnapChat users.

If this is the case, it's not as damning for SnapChat as it seems, at least until some younger, hotter social app bumps them off the pedestal in a few years. Young users get older, but older users don't get any younger. My understanding is that SnapChat's inscrutable UI is deliberate: they want you to have to be shown how to use SnapChat by someone already on it, to avoid the "My grandmother's on Facebook, how uncool is that?" problem.