Sean Taylor at Facebook took the approximate 35 million users who have "Liked" a National Football League team's fan page, and put it into this awesome map.
Taking this from a Steelers fan perspective, this map is particularly interesting. As expected, Steelers fans dominate Western and Central Pennsylvania, all of West Virginia and significant parts of Ohio. Perhaps not as expected is that the Steelers appear to be the most popular team in Hawaii, large parts of North Carolina and South Carolina, and areas of Alaska, Oregon, Nevada, Idaho, Montana, Utah, Nebraska, New York, Kentucky, North Dakota, Tennessee, Virginia, Alabama and Florida. Since I love maps, I really can look at this map for hours.
Facebook also went a step further. As Barry Petchesky of Deadspin writes, "If you're a fan of one team, who are your friends most likely to be fans of? Here are the top five most common allegiances for Facebook friends of fans of each team. Once again, geography rules—except for the large numbers of Cowboys, Steelers, and Patriots fans on everybody's list."
I hate to admit it, but based on this information, it appears that the Cowboys truly are the most popular team (at least according to people on Facebook) in the United States.
Update: I really can't stop looking at this map. Maybe it's because I am part of the Steelers diaspora, but I am fascinated by teams that have little to no support in places outside of its own vicinity. (I'm looking at you Jacksonville, San Diego, Tampa Bay, and yes, even Baltimore and Philadelphia.)