I am going to briefly compare two different Earthfury experiences to give more context.
I have a 53 lock main and a 38 shaman alt. My warlock has about 1 whole level of rested due to my playing my shaman with a friend (my warlock will be my main at 60).
I took my lock to Searing Gorge yesterday around 9 pm EST when I got home from work. It took me over 30 minutes to finish two dark iron dwarf quests. I decided to just leave and do something else. The mobs were camped. There were a ton of both alliance and horde around. It almost reminded me of Herod.
I am a PST player. I play late at night often (till 2-3 am PST which is 5-6 am EST). I have been running SM with a core group of a priest, warrior, and shaman. We have had to 3 man SM library and 4 man SM Armory the last two nights due to challenges filling out a group after 12 a.m. PST. A /who Scarlet Monstery last night at 2 a.m. PST showed 4 people in SM (our group).
But the server LFG still has people looking for strat/school/brs groups at this late hour. Part of it is probably that the more hardcore players that stay up late have out leveled SM.
So what does all this mean? The population is very healthy during prime hours. The population dies out late at night. The higher end content is more active later at night. And frankly, all of this is like Vanilla where even high pop servers lost a lot of activity very late at night.