As said here, mainly EU server's so mainly only EU player's playing, it's the same in AUstralian community, they haven't transferred there server's to xonotic yet and I think the one they had is currently down still so they are not playing online at the moment.
As for the US, most of the server's haven't been swapped over yet, even in EU we have this problem but we have a few player's atleast set up.
This is the main reason for lack of player's as well as it only been a preview version.
When 1.0 is released, expect a mass migration of server's, more promotion of the game and also a lot more of the nexuiz player's coming over (as server's move, player's will) as they find out more about the game.
So you'll probably need to wait till 1.0 is out, but I hear that US server's should be getting set up in Xonotic soon.
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