A surprising private study this spring found South Dakota has the highest Facebook adoption rate of the 50 states, with 31.1% of us having accounts on the social networking site.

But that doesn’t tell us how active we South Dakotans are on Facebook. Consumer research group Experian/Simmons issues its 2010 Social Networking Report and finds us about average, far from addict-level in social networking logins:

Simmons Heavy Social Network Index 2010

The Simmons report website doesn’t explain the “Heavy Social Network Index,” and their color-coding appears to be off: the middle-range white region on the legend appears to be light pink on the map. But Simmons appears to show South Dakota middling in heavy social network use, about the same as rural places like upper New England and panhandle Oklahoma-Texas. There appears to be slightly more avid Facebooking in areas of Minnesota and North Dakota. The biggest blob of heavy online social networkers runs through the Rockies to the Pacific Northwest.

Perhaps the most interesting place to study on this map would be Texas, a state with regions falling into each of the five index categories, from way high to way low. What cultural factors might lie behind those differences within a single state?