I believe the difference is that when Digg screwed up the redesign Reddit was ready to take its refugees. When the Reddit fiasco happened last year there was no clear place to migrate to.

Having said that, and being perhaps too negative, do we know how alive Reddit actually is? Because once you factor the power users that left, the bots reposting top comments on reposted stories, the auto-generated content and the shills it is possible that Reddit has already started its decline but hasn't noticed yet.