ZPS is weird, because it's basically a well known underground mod, to put simply.. People from hundresds of ountries play, but only a few six hundred or more play at one time. The server is most easily populated from 12pm-5pm EST, I've noticed. It becomes harder as night comes on but I've filled servers at 2am in the morning with foriegners..
Best maps to populate with when alone:
zpl_armory2
zps_cinema
zps_smashcannonz
zpo_shreddingfield
any deathrun
You can essentially go afk for 30 minutes on any of these and usually one to four people join. Armory is the best though, you just have to make sure any players tat join don't leave immediately because you're afk and nobody joins. Some people patiently wait for more joiners, preferring emptier servers.
there's just something about armory that attracts players, I'm almost certain that the erver being on armory makes it higher in the server list when they're looking for a server to join. I think this because we get SOO MANY first timers on armory it's retarded. If you ask them why they picked our server, they either have no idea or say it was one of the first ones. Even when I was the only person in the server I've gotten new people like that.
so while most of you hate armory, maybe you'll understand my love for it a bit more. You also get some nice, quick target practice.