This should not surprise anybody, but by one metric, the San Francisco 49ers are the most popular team in Northern California. Yep, the team that has been to the last three NFC title games is more popular than the Oakland Raiders. I know you are shocked and amazed by this revelation. I'll give you a minute to pick your jaw up off the ground.
The folks at Twitter like compiling all sorts of random information based on who people are following. They recently put together a map of every county across the country, focused on people following team twitter accounts. Twitter allows for GPS tracking of where people are tweeting from (Terminator is clearly well on its way to happening), and they can see where a lot of fans are located. I believe you can disable this function, but for now, it has created this detailed map.
You can play around with it and see where the 49ers are most popular. Obviously Norcal and a sizable chunk of Central California are 49ers country. There are numerous random counties around the country where more people follow @49ers than any other team. There are several counties in Nebraska, including Sherman, Custer, Garfield, Loup, Wheeler and Valley. Somehow Wilcox County, Georgia is led by the 49ers, then Raiders then Cowboys, but is entirely surrounded by Falcons fans. It's odd, but amusing.
Play around with this. In terms of sheer land mass of fans, the Seahawks probably win because they get Alaska. But the 49ers are spread fairly well. It declines as you go into the northern part of the midwest and then the northeast, but that is to be expected.