The map displays some interesting if not unexpected trends. There are pockets of Red Sox and Yankee fandom outside of New England and New York, for example. On the contrary, the Mets and Athletics aren't even the most popular teams in the region in which they play.
Over at fivethirtyeight.com, Nate Silver took a different approach to analyzing team popularity. Using data from Google Trends, Silver graphed the relative search popularity of each of the Major League teams (Google Trends aggregates similar search terms like "St. Louis Cardinals" and “Saint Louis Cardinals). Here's Silva's resulting graph:
Based on the data, the Yankees and Red Sox are searched for 3.8 times and 3.7 times more respectively than the average teams, and 10 times more than the least popular teams. Silver goes on to compare the searchability of each team to the size of the television market in which it plays, and the resulting number suggests the Red Sox "are a clear No. 1 and are about three times as popular as you’d guess from the size of the Boston media market." Pretty interesting stuff.