It's unusual for an established site to grow so fast. This is in part thanks to digg's fumbling mistakes, but is no doubt due to larger trends too.
It'd be great to have a more in-depth post from the Reddit crew about what they did to engineer such growth, as the HN comments are, for once, not that helpful, but in either case, one take-away is that Reddit is now the source of social news traffic par excellence. If you can leverage that enormous and hyperactive community, you can grow anything.
StumbleUpon, Facebook, Youtube and Twitter are still larger drivers of traffic than Reddit, but then it's arguable whether they are really social news sites.