I looked at the facebook network for La Crosse, WI, my hometown and was impressed that there are 12,856 people on this network alone. People on facebook can belong to only one regional network at a time, so its likely that most of these users actually live in the La Crosse metro region. The city of La Crosse has a population of 51,818 and the metro region has a population of 126,838.
Surrounding cities, like Onalaska, WI don't have their own facebook network, so people living in these cities are no doubt in the La Crosse network. Regardless, this means that 10% of the population of Metro La Crosse is on facebook.
La Crosse is not extemely tech savvy, but 10% is much higher than the New Orleans, LA facebook score. With 42,789 facebook users in the New Orleans network out of an estimated 1.2 million metro New Orleans population, New Orleans facebook score is a 3.8%.
San Francisco (city, not metro area) scores 15.4% (114,792 users of 744,041 population) and New York is 5.1% facebooked (425,963 users with 8,214,426 population).
It would be an interesting mashup to take facebook network membership numbers, us census data and put them on google maps.