The game doesn't seem like it's 'depopulating' to me - I still tons of people around in Revenant's Toll or the three cities, obviously the time of day has a lot to do with it as well, there will always be 'peak times' when most players are on and 'quiet times' when most players are offline, usually sleeping.

Even FFXI was like this - in fact FFXI was worse where I was used to large periods of time when zones were empty for the reasons I've already mentioned. Server populations are not static - they fill and empty with the clock.