The New York Times has an interactive baseball map that you can zoom in and view fan allegiance down to the ZIP code level based on FaceBook "likes". It's not much of a surprise that most Padres fans live in and "like" in San Diego county.
A Map of Baseball Nation - NYTimes.com
Facebook data reveals the most popular team in every ZIP code.
I can see this map being beneficial if you're looking to move around town. I'd want to know that I'm not moving into an area where Red Sox hold more than a 5% foothold in the community. Currently I live in a 54% Padres area with 8% Yankees and 8% Dodgers. That suits me just fine.
Looking at the map I'm surprised the Padres don't have more fans in Yuma, you'd think that all those years of playing Spring Training there would have laid some foundation for the future. Maybe Padres fans that are there are old and crusty like me and don't use the Facebook.
As a segue I'm going to try and embed a link to the Gaslamp Ball Facebook site so you can "like" it. The GLB Interns are doing a good job of keeping it updated with unique content.