First of all, a game doesn't need a million active players to sustain itself. Reach still has over 100,000 people playing daily, which is pretty good in the week following a Call of Duty release.
Secondly, threads like this tend to see the most activity early on. Reach has been out long enough now that people have sort of finished discussing it. Occasionally someone will come in and bump the thread but you won't see a flurry of activity. Also, Call of Duty tends to attract more activity because more people get it, it's available on more platforms, and there's more customization.
Anyway, I don't really care if CoD has more people playing, I'm still going to enjoy Reach as much as ever. Hell, Halo 3 is still pretty popular. You can even go online and find a game of CoD 4 if you want, although it'll take longer especially if you want one of the less-played game types.