The only thing about the leaderboards is that you really need to look at the 'all time' stats, rather than Weekly/Monthly. Whilst the Weekly/Monthly stats should be accurate, its only a snapshot of how you have played over the past few days/weeks.. And just incase you havn't noticed, in some cases they are NOT accurate and as such Treyarch keep resetting them...
So you really need to use the 'all time' stats, to get accurate data for each game mode... BUT.. Before you appear in this list, you need to play 50 games first.. So its very likely that they won't do this for quite a few hours play and on some game modes, never!
Personally I am not even on the FFA leaderboard yet, as I have only played 39 games...
Jon