Honestly I have never been an active member of a webmaster/admin forum. They were quite popular back in the day, however they, like all forums seem to slowly be dying out. It's a shame.
I'm in agreement with Kino. Admin forums used to be pretty big and booming. As of late, I've yet to find an active one that I enjoy enough to make posts. They all seem to replicate all the others. Even the members on every community seem to be the same. It's annoying to me, really.
In the past, ForumPromotion has always had 2-3 really active admin/ promotion forums advertised here (by members who have created them). That is no longer the case.
A significant number of people now post to Facebook groups rather than forums even though, in my opinion, there are massive advantages that forums have.
I'm currently running, owning and managing one, so I'd like to personally say that no admin forums aren't dying. If there were dying, then theadminzone wouldn't have a ton of different posters making over 50-300 new posts on a daily basis.
Forums are only active when they have active staff. I've visited a couple of forums and seen how many users posted problems they've been having but no one to answer them. When I join a forum I want them to have dedicated staff. I expect constant activity from the staff's end.
I've checked out a few admin communities, and sadly, they're not all that active. I really enjoy the nature of discussions relating to management of forums. You can learn a lot from others. I think they have at least some potential to succeed, but you need to put the effort in. ^^
I don't personally use them nowadays, I just lurk and check them sometimes.
Admin forums are typically made up of people who run their own forums so you'll see a lot people who are being condescending and patronizing towards each other. Too many chiefs and not enough indians. Also the topics of discussion on admin forums tend to be a little dry and overdone.
It can be hard to sustain a community like that, so that's why I think a lot of them reach stagnation pretty quickly.