The number of users online has never really surprised me, specially if you've looked at the amount registered.
Not to forget that the more people that hang out in other maps than Fluorite, the more people will come online, I suppose.
Yes, then we've got the timezones. It's rather usual that it's a lot of users online when it's late where I live.
The more users that play this game nowadays, the more will find out about it.
People will tell their friends and family, maybe upload pictures here and there,
something which will cause others to go "Hey, what's this game? It looks cool."
I've had people commenting on my dA asking what game I'm playing, where and how to get it.
It's always during 12:00 to 7:00 where lots of people are playing, but it soon all dies down at night and stays low until the next afternoon. Ya know, school nights, homework, work, stuff people have to do and Christmas parties or shopping and all. It's all like a routine. xD