Is there a consensus about what caused the decline of Digg? I thought it had a lot to do with the redesign that couldn't be reverted, but I don't like to peddle hearsay and assumptions like it's the truth, especially since I never used Digg.
I know they suffered a big drop when it came out that they were manually curating the links on their front page. I don't remember the details, but that's what did it for me.
IIRC the redesign was the final straw for many users and caused the mass-exodus. I think the functionality of subreddits and the ability to completely ignore the "mainstream content" if you so wish has been the main feature that has helped reddit stop the decline.
Subreddits are what will keep Reddit strong. Even though Reddit has tons of members, subreddits make it feel much smaller and personal.