I've been told by my friends that the activity on halo 3 has gone up quite a bit due to halo 3 coming to xbox live for free download soon, so there are going to be TONS of people playing halo 3 once that hits.
I play every once in a while as well. I thought they were going to be giving it away for free to Gold members, which would've boosted the amount of people playing, but they never did. Mostly just mess around in social because the people in ranked are the ones that never stopped playing and I'm very rusty