The Thinking Atheist > List of Shroud Discussion Threads (below the ads)
JREF Forum > List of Shroud Discussion Threads
Catholic Answers Forums > List of Shroud Discussion Threads
Catholic Answers is the biggest of these forums with about 400,000 members and over 750 shroud related discussion threads, many of them fairly current. But, for the present, JREF has the biggest, most active thread (probably ever) on the subject. Most threads on most forums die naturally and quickly. Not this one JREF thread: Miracle of the Shroud / Blood on the shroud. It started in December 0f 2011 and continues even today with well over 8000 comments and 350,000 views.
Our very own Rich Savage, Hugh Farey and Colin Berry are participants there. Colin is a brand new member and can be read starting at comment 8011.
Virginia Hefferman has an interesting perspective on forums in the New York Times. If she looked at JREF and Catholic Answers she might change her mind.