But their algorithms for aggregating are very different. Reddit is primarily based on submitted links. Digg is primarily based on links shared on social networks.
True. However, at the end of the day, both of these sites revolve around the content "submitted" by users (either directly on the site or across social networks), and they need this content to thrive/survive. So while their purpose/methods may be very different, both sites co-exist in the same ecosystem?