Much as Reddit now has these impressive numbers, I think they have issues of their own with their new community. Before the Digg fiasco their numbers were lower, but the community felt settled and involved. The surge in numbers eems to be mostly kids, who can be very fickle in their online habits. You don't have to hang around reddit long to see how shallow and repetitive the content has become, which isn't going to keep people around for the long haul.
Mind you, the subreddit system means there are still pockets of worthwhile discussion. Reddit may be very glad of this bedrock community in years to come.