13. The system of claim 8 , wherein said program code for determining whether said sub-thread is off-topic comprises: program code for determining tag rankings of said user-defined tags, said system generated tags, and said moderator-promoted tags; program code for determining a conjunctiveness score of said selected post with respect to its parent topic post in said discussion thread responsive to tags associated with said selected post, said parent topic post and said tag rankings; and program code for determining that said sub-thread is off-topic in the event that said conjunctiveness score is below a predetermined threshold.
13. The system of claim 8 , wherein said program code for determining whether said sub-thread is off-topic comprises: program code for determining tag rankings of said user-defined tags, said system generated tags, and said moderator-promoted tags; program code for determining a conjunctiveness score of said selected post with respect to its parent topic post in said discussion thread responsive to tags associated with said selected post, said parent topic post and said tag rankings; and program code for determining that said sub-thread is off-topic in the event that said conjunctiveness score is below a predetermined threshold. 14. The system of claim 13 , wherein said program code for determining said tag rankings is responsive to frequency of use of said tags in said parent topic post.