Hi Matt,
Thanks for your interest in the ACS Online Community. I can't speak to other options for user groups or listservs, but I will mention that there tend to be more user posts in the fall, around the time of the Census Bureau's ACS data releases. Also, certain discussion groups--such as ACS data for small geographic areas--are more popular than others.
We are going to be upgrading the ACS Online Community to a new platform over the next few months, which will create some new functionality and improve the user interface., so this may increase use.
-Mark