I have visited several forums daily since January 2005 when I first found the MR forums over at Kalmbach. Each place has a discernible culture, to me. Each has a pace, and in each case, the pace varies seasonally and with no apparent relation to the time of year. I have seen this forum busier than most, but not since this summer. Three years ago this was one of the busiest. Trainboard seems to be perpetually slow. Nice solid folk though. Railroad Line is also slow, but the caliber of skills there, and the evidence posted, are astounding. If you want to know what's happening among the elite in the hobby, try an hour or three there one Sunday. Be sure to go into their photography forum and peruse the monthly themed threads.
I still drop into Atlas Rescue, Big Blue (the Gauge), and modeltrainforum.com (high concentration of Canadian members, perhaps it IS Canadian).
The fact is that those of us who provide the daily fodder for the more popular forums are few in number. We get bored, sick, turn to something else for a while, die, sell, move, blow up computers....the list is endless. As a population, we are losing ground to the years. My guess is the typical rail forum has a mean age of around 65, whereas it would have been closer to 50 fifteen years ago. Just as steam is losing ground over time, the age of the hobbyist is getting older, and the electronic devices and media favoured to pass time with our youth are of a different nature. Trains don't figure largely so much in people's lives, unless they are blocking traffic somewhere or burning tank cars. Or steering into young high schoolers walking along the tracks with their ear buds firmly planted.
You will find that, if you polled the membership for an entire month, you'd find only 40? regular posters from day-to-day. Those few will generate 85% or more of all posts daily. The rest are shared among newbies, especially near Christmas and the New Year, and the odd guy who needs an idea or some advice about something. All forums seem to go this way over time. The hobby is populated with old geezers who don't spend time at a computer, who don't need/want a smart phone or tablet, who have aging cars needing some tinkering, aging homes needing upkeep prior to the couple needing to sell, and other ways of using their waking hours, plus whatever hobbies they have. So, the representation of your typical hobbyist is only somewhat approximated by the few regular posters you'll find at any one forum on any day you choose to do the looking.