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.
