Their revenue model is: (A) one AdSense ad per page, and (B) rewriting URLs to append their own affiliate link (if an affiliate code does not currently exist).

Their revenue model is: (A) one AdSense ad per page, and (B) rewriting URLs to append their own affiliate link (if an affiliate code does not currently exist). For the record, Delicious alternative Pinboard also has a revenue model, whereby they charge each new user a certain amount, and then also have a premium offering whereby bookmarked links are archived.