The report "American Nones: The Profile of the No Religion Population" from the American Religious Identification Survey shows that a growing percentage of the population list no religion. Barry Kosmin, the lead researcher, projects that perhaps as many a 20% of Americans will be religious "nones" by 2030, up from the low teens now.
However, very few (7%) are atheists. Most are skeptics. They seem to me to be skeptics of institutions even more than of theology.
I have noticed in other research that unmarried people tend to be less attached to other institutions, as well. The nones are 39% unmarried, compared to 25% of the adult population as a whole. The nones are also much younger than most Americans, and many of them will affiliate later. Still, even adjusting for age, the nones are 33% unmarried, compared to 28% of their age-adjusted cohorts.
Elizabeth Marquardt found that children of divorce are less likely to affiliate with religious institutions. This report does not show the marital status of the respondents' parents. However, children of divorce are more likely to put off marriage, which is one reason that the children of divorce tend to show higher proportions unmarried at every age. I think it likely that the nones are disproportionately the children of divorce. They are skeptical of many institutions of traditional adulthood. But they don't reject the belief that lies behind them.
I think a large proportion of the religious nones are institutionally disconnected. When they find a way to connect to one institution, they are likely to connect to others, as well.