Aside from the diehard few,I think any forum membership has a shelf life usually defined by flagging interest and the need for pastures new. You'll always see folk coming and going as life events dictate and otherwise make demands on an individual's time. If you get a couple of years of a member consistently posting,I reckon that's the tail off point for many.
There is also the unfortunate reality that some members just run out of material,sometimes it's a dry spell,sometimes it's over. When I see someone posting a back catalogue from years ago,I tend to think there has been nothing new since then. How often can a member do that without wearing out their welcome?
Using myself as an example,I did no writing at all for the guts of 20 years and channeled my energy into,in no particular order,drinking like a fish,motorcycles,playing live in a band and frenetically energetic sex sessions,albeit often solo,but I had to slow down,it was making me walk funny.
If the forum has maintained a changing nucleus of active members,even if the number seems relatively small,then surely that's a sign of good health? As Boydie mentioned,it would be a very different place if we had hundreds of posts every day and whether that would be beneficial to what we regard as a community,is in question.
I don't know which members have achieved any level of success,I suppose it depends on how you define 'success'. I know a few,who seldom post now, have their projects running,which must be described as successful? Speaking personally,I don't put any work into chasing publishers or hawking my wares,but I'm always glad for those who take a step toward their own dream by gaining some recognition for their work. It would be a bit mean spirited not to be delighted for them.
The sun has been shining here all week and I haven't even turned on my Mac to do any music stuff,I've been biking,cutting trees and generally doing outside stuff. I'm loving waking up every morning to a blue sky,because I know it won't last.