The bulletin board (which I define as the collection of topic-specific fora) belongs to a large non-profit organization, and is part of a large web site, most of which is dedicated to directly serving the purposes of the organization. Every page on the site (including the bulletin board) has links to all the main sections, so in theory the bulletin board could attract surfers who would later go to the main part of the web site.
I suspect that few did. My feeling is that the bulletin board attracted its own crowd, more interested in the specifc information exchanged than in the sponsoring organization.
And the off-topic board generated enough garbage that the webmaster decided to ax it, finding the cost (staff time) needed to monitor it too big. If off-topic posts start being made on the other fora I believe that the whole bulletin board will be shut down.
How do the many successful bulletin boards that operate in this specific niche manage to thrive? All are standalone bulletin boards, operated by individuals for the fun of it. They therefore are willing to invest the considerable time needed to keep the fora healthy.
I believe that the message of this story is twofold: