The "users ceasing to post stories after one year" metric seems a bit wonky to me. I mean, I don't doubt it, but it's probably skewed by accounts created for self-promotion or spam. If you look at front-page stories, I think that most of them are from long-time members.

It would probably be useful to separate the churn of throwaway accounts and the ones that cross some engagement threshold - 50 comments or something like that.