I'm a huge soccer fan but it's not the "4th" sport in the US yet. I know the traditional top 4 sports are football, basketball, baseball, and hockey. But this was largely made by ESPN (who didn't cover racing much in the early days). I believe that now, in terms of TV viewing and number of "fans", NASCAR is #2 in the US, behind only the NFL.
NASCAR is an interesting case though, because many of the non-fans don't call racing a real sport. Also, not just anyone can go out and race cars, but anybody can go play basketball or touch football or soccer or whatever.
Soccer is definitely growing in the US, and now record numbers of youngsters are playing soccer. Whether or not that turns into increased popularity among adults is yet to be seen, but like many of you have said, it hasn't happened yet. It is also possible that if the US national team improves (they are quite respectable in the world already), then soccer will increase in popularity amongst normal americans.
It's gonna be touch to nudge their way into the major TV contracts though, since we as Americans already have the NFL, NASCAR, college FB and BB, the NBA, the MLB, and many other popular sports (even poker gets a lot of viewers). In most other countries around the world, soccer is the number 1 sport, and the others are distant competitors (with the possible exceptions of Formula 1 racing and cricket in like India and Pakistan). They don't have as much competition, and certainly not as much well established competition like the US does.
p.s. - The main reason we lost in basketball is because our players never practice as a team and don't play as a team. The European and South American teams beat us because they play much more team oriented ball, much more similar to college basketball than NBA. Our players are much more talented, but they don't play team ball......which is another reason why the NBA has fallen in popularity in the US.