So do I. It seems my whole adult life (15 years), I've seen exploding, new nondenominational communities, but then I go back up to 15 years later, and there are just new nondenominational communities, but no more nondenominational Christians than there were before. I suspect many of their congregants are dissatisfied with their former denominations, and like being able to pick the congregation that believes what they want to believe, but then, after many years, there's really not that much to hold them in.
I know so many non-denominational churches claim that a majority of their members are ex-Catholic. Well, guess what? Catholics are pretty close to a majority of registered Christians in the country, so for a congregation made up mostly of young converts, that makes sense. But I can also tell you that a few of the parishes I've lived in have a very vast number of former former Catholics! Typically, they dropped out of the Catholic church, not having learned much in the dreadfully weak-minded modern CCD books. Then, knowing very little about Jesus or their faith they were attracted to the people making the most noise. But then, once they started doing a lot of their own religious formation (scripture-reading, etc.), they realized that they simply hadn't understood the Catholic faith very well.
Of course, this is all based on a not-terribly-broad personal experience. I've lived in enough places I feel I have a little insight into the religious practices of many Hispanic immigrants (of course, being gross generalizations!). But I haven't lived in the non-denominational "hot-spots." Plus, it's my understandin that a vast proportion of immigrants end up in nondenominational churches, and gosh knows this place is flooded with illegal immigrants. And in cities, sometimes you can't tell a nondenominational church from a check-cashing service or phone-card store unless you happen to walk by during one of their services and hear the music! (This is a reflection of their poverty and a contempt for "trappings", I believe, and is no sign of a lack of reverance.)
I would be very interested to know how many people attend non-denominational churches; but by their very nature, except for the megachurches, they are very difficult to enumerate.
The big growth in the non denominational church was during the Charismatic renewal; many left their churches and went non denominational due to the fact that they were Spirit filled. This is particularly true in the Catholic Church, it seemed like the more ridged and formal the belief system the greater the flight from that system to the nondenominations.
According to Barna, 4% of the US pop is nondenominational or independent. Or about 12 million people.
I live in a small town and have watched the non-denoms form, split, die, regularly and you are right, it is essentially the same people just trying to find a theology that fits them.