You can find people to play with on public server all the time, however I think most of those are hosted it europe so YMMV. The main problem I have with Warsow is that the greater part of the regular userbase is very experienced and when you try to get into it at first it can be a very... punishing experience.
It's very fast paced and requires quite a lot of skill and muscle memory to become good at it. There are a couple of "noob" servers that are quite enjoyable however. Overall a very good game.
Just so you know, UrbanTerror isn't open-source, sadly - still very good fun though.
There's a pretty fun FOSS FPS called AssaultCube which I used to play a lot - it's simple but it's fun. That has quite a high number of players. Unsure about how active the other FPSs are but whenever I've tried them out I've always been able to get a game.
Ehhh the public user base perhaps isn't really so important in foss fps games because these games are most often used as LAN games when not everyone owns the same collection of games. Before tf2, Warsow was pretty much the only good fps LAN parties could use legally.
Xonotic and its predecessor Nexuiz are also 100% free and open source (git.xonotic.org). Even the media like models and sounds are also released with source links, which considerably different from what Warsow is doing.
I don't think that's small - OpenArena has enough players for me, that's probably less than 4000-6000 a week. It's actually kind of nice to know some people on every server you go. And being anonymous is easy, just rename to unnamedplayer or some random name (the default name).
