Tough question.. as I roamed only international forums I rarely visited any local forums so my grammar on this is limited, but let me think..
Well, post is: הודעה
Thread is: שרשור
That's easy.
I would go with the word: צביר
The direct translation is "cluster" but personally it sounds fit to describe collection of forums and sub forums.
Just let him know it's not official, just idea of one person.