The New York Times recently posted a detailed look at Madden 11 and the usage rates in online play of every NFL team. The data is tracked from the start of the season up to the Super Bowl and shows trends and some points of interest explaining any notable shifts in usage.
The Eagles finish the season as the most popular team online which is no surprise considering their explosive offense. Despite down seasons the Vikings finished #2 overall and the Cowboys #4 (though they did lose some support as the year progressed). As one would expect the Packers and Steelers skyrocketed during the playoffs.
By far the biggest drop from the start of the year was with the Bengals who began at #4 and ended up at #28. Also make sure to check out the influence that Randy Moss had on the Vikings and Titans. Both teams saw huge spikes following his arrival only to sink drastically after only a few weeks as his performance tanked and respective teams struggled.