Digg had a large viewer base and there was a lot of skullduggery going on amongst people who figured out how to game its algorithm, get on the front page, and direct traffic to some URL. But without actual data I would venture to guess that Digg and Reddit had roughly equivalent bases of actually genuinely active community posters and commenters and a lot of people were on both. Once Digg got taken over by the spam posters, it died off and Reddit remained. Reddit definitely inherited its mantle and probably many community members, but not the massive viewer audience.