The Major League Soccer website has an article in which Dan Hunt, the president of Hunt Sports Group (which owns FC Dallas), speculates on the prospects of MLS surpassing other American sports leagues in popularity:

While Hunt isn’t prepared to say that MLS will be on par with the NFL in terms of popularity in the next few decades, he does believe that Major League Soccer could be right behind it. “Up until [the early 1990s], baseball was the most popular sport in the United States and then the NFL has since sailed past it,” Hunt told MLSsoccer.com in a recent interview. “With the growth coming in soccer, it is very quickly going to pass a lot of these other leagues.” Average MLS per-game attendance has already surpassed the NBA and the NHL, but a wide gap remains between what the traditional big four of American sports command in television rights fees and what MLS pulls in. Hunt believes that international success by the US men’s national team as well as by MLS clubs in CONCACAF Champions League will be key to broadening interest in MLS amongst fans that don’t already follow the league.