I was looking for a site where it'd be popular in Montana, Alaska, Dakotas, but not California. I hit the jackpot. I will post the answer as an update to this post…
Discuss ;-)
UPDATE: Too easy! Randal correctly guessed that it was Adultfriendfinder. I imagine the reason is that unlike California, there's fewer people in Alaska and Montana, and they use more dating/adult sites. That's the theory at least ;-)