Hmmmm is the overall membership and posts on the decline? I am not around daily like I used to be as I am currently without a Mustang in my garage so I don't stop by as often as I should these days.
I know this is off topic but want to try and answer your observation about membership.
Activity has waned since the peak S197 time frame. And there are other Mustang Forums with the equivalent activity which tends to bleed membership. But I think the drop in activity level is related to migration to other brands. No secret I have a 4 door Charger scat pack, and at 485 hp and 475 torque it is every bit as exciting to drive as my Mustang. As you can see by the sales numbers below, the Charger is outselling the Mustang so far this year. And Camaro sales are starting to lag which probably answers why I couldn't find sales data for them for April and May.
The Challenger which most of us Mustang owners view as the competition along with the Camaro, lags Mustang in sales, but when the Challenger sales are combined with the Charger sales in the R/T 5.7L, Scat 6.4L, and Hellcat 6.2L models it all makes a formidable sales challenge for Mustang which also speaks to forum membership across all Mustang Forums.
MUSTANG: 75,842 (2018). 32,203 (2019 thru May)
CAMARO: 50,963 (2018). 11,792 (2019 thru March)
CHALLENGER: 66,716 (2018). 23,601 (2019 thru May)
CHARGER: 80,226 (2018). 36,012 (2019 thru May)
What Ford is doing, or plans to do to increase sales is a mystery to me.