With the inevitable change in the demography of the country, soccer (MLS + Premier League combined) will be ahead of the NBA, MLB, and the NHL, but the NFL will still reign supreme, not least because of the many commercial breaks that allow it to pull in big revenue and thus ensure prime time air on the major networks. Soccer has no stoppage of play and while this is great for spectators, it's not so great for bringing in cash money.
Still, NBC is paying $100 million a year to broadcast English Premier League games here. Keep in mind many of these games air before noon. NBC pays the NHL $200 million a year for broadcast rights, and that's a domestic league with a season twice as long and playoffs too. MLS is coming along, but it's a long way away from being the quality of the top European leagues. Personally, I watch the Premier League/Champions League religiously but rarely watch the MLS, and the domestic league's pittance of a tv deal suggests I'm not unique in this respect.
That said, MLS is growing and has done a great job from a business perspective. If the quality of the league improves (and that means bringing in promising young foreign players, not big names looking for an easy paycheck before retirement), then I will happily convert my fandom.