Just looking at some figures out of interest.
This forum currently has 1,827 active members. Active members are those who have visited in the last 60 days, regardless of what they did here.
3,175 members have made 10 or more posts, though of course some of these could have been years ago.
Total members is 21,544 (that's everyone who has registered other than those banned, retired or RIP), but that is not a useful figure. Some of these could have signed up many years ago and never posted.
The BVWS currently has between 1,200 and 1,300 members (I don't have the exact figure). These would obviously have chosen to renew and pay, or would be complimentary or honorary members. I think the peak was around 1,800 but I'm not sure.
The Facebook group for this forum
https://www.facebook.com/groups/59466401554/ has 795 members.
The BVWS Facebook page
https://www.facebook.com/BritishVintageWirelessSociety/ has 525 followers.
On Facebook a page is more of a place to show and promote whereas a group is more interactive, so these don't compare directly.
From the information available to non-members the Golborne forum has 443 members and Vrat 2.0 forum has 129 members. I am surprised those aren't higher. Radio Museum claims to have 16,859 members but figure is probably about as meaningless as the 21,544 members figure here.
These figures can obviously be interpreted in different ways depending on what you are trying to prove. There is nothing that is directly comparable.
It does seem that people prefer the more immediate interaction of a forum, and that will inevitably be true more for younger people who have grown up with the internet and smartphones etc. Most online stuff is also free of course.