Maybe index all content into a Lucene index (either using Solr or Elasticsearch) and when someone has written a post do a "more like this" query to find out if there are any highly similar posts. (that would need to be tuned carefully to actually bring back similar things) Also, when searching, instead of using this google site search, using Solr or Elasticsearch. Write a relevancy boosting function to take into account the date it was published, favouring initial posts on the subject (maybe?) as well as taking in to account the popularity of the post in the relevancy boosting function.

Some ideas from someone working on search.