Digg's userbase collapsed so precipitously because there were alternatives already in competition (reddit itself, largely).

There isn't really somewhere "quite like reddit" enough for reddit users to march off to en-mass in the same way, though I, people I know, and I suspect a large number of others, visit reddit a lot less than we once did and hang around less once we are there. So rather than an exodus there is more an epidemic of "meh".

I certainly don't contribute to "communities" there these days - and I think many others with a similar attitude is why some significant subs have fallen to rabid fighting where at least some insightful discussion once prevailed: the more reasonable voices have shuffled away so the more rabid ones increasingly stand out (and boy do they love it), pushing more "more reasonable" voices into shuffling away leaving the polarised trolls on all sides to fight amongst themselves, and so the spiral descends.

Though I'm not an ad-clicker, gold buyer, or anything else such, so I very much doubt the powers that be over at reddit care that I am not engaged.