I am not sure what the main driver in the decline of forum membership may be, but it could be at least one of the following factors:
1. Inaccessibility. The site may not be accessible as easily as the old boards.
2. Problematic users and spammers causing trouble.
3. More than one message board devoted to game shows. G-R.net is devoted to The Price is Right, and I thought I read somewhere that posting activity on that forum is down as of lately, but not as significantly as GSN's boards.
4. Facebook. I noticed that RealityTVRules.net was shut down within the last few months in favor of a Facebook page. Plus, membership on RealityTVRules.net went down significantly, as there were so few new members in the last year of business. Facebook is a site where people meet and is likely one of the reasons why some online message boards are heading the way of the dinosaur.
Worst case scenario, if the GSN boards disappear completely, all I can say is, it was a wonderful ride, that's all I gotta say.