In terms of audience/fans, football is driving the sports bus in this country right now. The NFL has drawn out its schedule so much that football is now a 12 month a year activity. Regular season from September to January, playoffs through January and Super Bowl in February, free agency starts in March and scouting combines in March, draft in April, mini-camps in May and June, training camp starts in July and the cycle begins again.
Soccer's rise in popularity in this country, IMHO, is directly related to the internet. It's one thing to be a soccer fan in the US and read about Lionel Messi and see him once every four years in a World Cup. Now a fan has the ability, if one so chooses, to see just about every Barcelona match. Every Premier League and FA Cup match is either televised or streamed in the US via NBC Sports, and with the right cable package one can watch Serie A, Bundesliga, and Ligue 1 matches, as well as Liga MX from Mexico. I don't think soccer will ever be as popular in the US as the other sports, but there certainly is no reason why it can't continue to carve out a niche.