Update: Digg shared some more details about how its search feature (relaunched earlier this year) works. Sammy on the company blog says:
" We’re using Apache SOLR/Lucene which helped us scale horizontally and solved many of our relevancy issues as well as enabling discovery of new content through facets. Beyond site search, the rich set of features has allowed us build a platform that enables other features such as Related By Source and Related By Keywords."