http://m.espn.go.com/general/story?storyId=9902116&src=desktop
I couldn't find the statistics for the total numbers but the above link is for high school per 10,000 exposures and you'll see that girls soccer is 6.7 compared to 11.2 for football. That's on an even scale, if you think about how many more kids play youth soccer than youth football in the U.S. then I could see how what I've heard is true.