There is a professional soccer league in the USA. In Seattle, they draw crowds between 25,000 and 35,000 on a regular basis, and they play in Qwest Field, the Seahawks stadium. It is not a fringe sport.
I think Seattle has one of the more successful franchises; we have the “Red Bulls” (the former Metro-Stars) and they just moved into a smaller brand-new stadium nearby. They’re struggling, but they should grow slowly.