Also, it assumes that usage of each category on craigslist increases at the same rate. Since housing and erotic services have strict separations between them on the site, it seems as if the network effects would be relatively (but definitely not completely) isolated to individual categories. Secondly, network effects increase the utility of the site logistically, so the ratios between different categories at a given point in time aren't meaningfully comparable.
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.
