Yes, soccer is growing ever so steadily. It has never grown by leaps and bounds in the US, but soccer has grown steadily over the last several decades. It is a grass roots movement if there ever was one. Interest continues to rise because people play the sport.
The real error is to perceive that our relationship as fans to any of these sports is in stasis. These relationships are in constant flux, as are all other cultural factors. Talk to someone from a previous generation. Baseball was once everything; horseracing used to out draw basketball by a large margin. Sixty years ago, professional boxing was much more popular than American football. Look at the rise of NASCAR over the last 15 years. How far have the X games come in a very short time? Things change.
My job forces me to converse frequently with folks from all over the world. The pervading and derisive sentiment is that Americans invent sports no one else plays so that they can proclaim their supremacy. I constantly defend basketball and American football to non-Americans, only to turn around and defend soccer to fellow Americans. Each defense is equally foolish. I recognize this even if I cannot help myself.
These are just sports, which are essentially war substitutes, a non lethal competition. Soccer is one the most popular cultural phenomenon in human history. It just is. That alone is the best defense of soccer. Many Americans are quietly and fundamentally disturbed that soccer does not appear to need them. Soccer will flourish, at least in the short term, whether America embraces the sport or not. Soccer does not need the US.