Depends on the game. Halo 2 Is obviously the most played game so you'll get matchmade in seconds. MechAssault is blooming, but will be cut off by MechAssault 2 shortly. Sequels kill off the originals most of the time, with the exception of Rainbow Six which everyone still seems to play here and there, despite Black Arrow being out for awhile.
Games like Dead Man's Hand which is old and didn't sell well, plus is being overshadowed by Halo 2, has about one or two people at the most playing on live. Then again theres a few games like Jedi Academy that still has about 40+ people playing on at peak times, and a few there at nights.
Out of all the new games I've seen released, Outrun 2 has the be the most dead live game out there. I never see more then six people playing it on Live. But people are still there, you'll meet a nice clique.