I played AOC for about 2 years before AOM came out, so I can give a little bit of perspective there.
You had to be in a room to play a game, but a lot of people just "hung out" between games. Sometimes they would have their name in a room, sometimes not, sometimes in more then one room. IIRC, there was about 3-4K people on most of the time. I don't know how many people were actively in the rooms at any given time, but you must realize that people often only stayed in a room for a few minutes or seconds to check it out. I would suspect that of that 4K people logged on, that at least 1/4 were actively searching for a game or playing a game (just based on the number of people actually in the rooms at any given time being around 1000.) I'm talking peak times here when I am able to play (weekend days and early evening hours, CST.)
With the zone, you had a zone-friends list that had your friends on it in a SEPARATE WINDOW. Thats really the key advantage of zf vs ESO. With ESO, you have to akwardly leave the room you are in to go back to your friends page to be able to answer one of your friends (without having to tediously type in /name before your response.)
Human nature being what it is, people are going to try to squeeze the maximum enjoyment from a situation with the minimum amount of work. With the zone, you could instantly respond to your friends (and multiple friends messages were queued so you could just answer them one at a time in separate conversations) and stay in whichever room you wanted to be in simulaneously. There was also a private chat room feature that allowed you to invite multiple people into a private chat room to have a real time conversation. ESO has public chat rooms, but there is no comparison to the desirability of a private chat room to a public one (for obvious reasons.)
I remember with the zone that it was not unusual for me to be in 3 separate conversations at one time within 5 minutes of sigining on. If I even talk to one person the entire time I'm on during a 2 hour period on ESO, its fairly unusual. I'm not saying thats necessarily a bad thing. Often times with AOC, I would rather have just played a game versus having to sit there listening to people yammer away. However, most people like the idea of having a group of friends that welcome them when they get on-line, as it makes the game much more personal. AOC was made of hundreds of little communites like that. AOM I doubt scarely has anything along those lines.
Thats a big reason why there are only 1500 people on at any one time, I think. The people that are on are there just to play and not to chit-chat. Actually, if you had a way to measure how many people are acutally playing AOM at any one time (not just logged on to ESO), I suspect it would be very similar to the total number that were actually playing AOC (not just logged in to zone) at any one time.
I actually, as mentioned above, probably prefer just playing more 1v1 games right now, so ESO suits me just fine. Thats more of a function that I'm older then most of the people online and have a little baby that sometimes requires my immediate attention (so I feel bad just dropping out of a team game and letting everyone down, but if I have to leave a 1v1, then the other player gets a win and at least some consolation there.) I've also not really into the usual topics that I've seen bandied about by ager's (porn, strippers, girlfriends, etc..) so more playing and less talking is OK from that standpoint.