USA Cycling Reports Higher Numbers

I think this says more about USA Cycling support (or lack of support) for mountain biking than MTB popularity:

"The sport of competitive cycling in the United States continues to grow, according to the latest figures released by USA Cycling ... Broken down by specific disciplines and types of licenses, road, track and cyclocross grew by 3.2 percent, while mountain bike increased 1.2 percent."

Via USA Cycling Reports Higher Numbers