This is absolutely true, however I did preface it with "The best I could come up with was the Housing category"

I'm completely open to a better metric. But as I said in the reply above, it's not easy to determine just how many people in an area use craigslist.

That being said, I'm not sure the two categories have such a hard seperation between the users. People on craigslist who use it for finding prostitutes probably also use it to find housing and vice versa.