At this point many people hang out in Mor Dhona or in their FC housing area so if you are gauging population based on how many people are in the main cities you might not be getting the full picture. A ton of people have started out on Hyperion and a lot of my friends who stopped playing have returned as well and I'm sure it's the same case for other servers.

Another thing to keep in mind is that during peak hours many players are probably in the middle of doing dungeons or raids so you won't see them around.

How active the server threads are is not indicative of population either. Many people are new to MMOs in this game, especially on the non-legacy servers, so they don't use forums much. It's only a small portion of the players that actively use the forums in the first place.