But the momentum isn't there. The 90% converts number was around when my mother was a youth raised in the church. While individual congregations grow, and some grow quite nicely, the vast majority of UU churches are small, and fission seems to be at least as common a trend than expansion. Of course, I'm basing that statement more on anecdotal evidence than on something like solid fact.
Closer to solid fact, we have this article from the latest issue of UU World magazine. The article does not detail the converts' experience, nor does it talk about the disparity between large and small congregations. Instead, it gives two numbers that would seem contrary: "675,000 adults in America broadly identify as Unitarian" and "The UUA’s 1,018 congregations in the United States counted just over 158,000 members". From this alone, it appears that we are shrinking. To underscore this, a survey among readers of UU World (who have to be members of a UU congregation) "showed... that only 12 percent were raised Unitarian Universalist". But this is all muddled - UUA congregations still attract a significant number of converts, and the faith is convincing enough for 500,000+ non-members to identify with. So what's going on?
The obvious point, and perhaps the most meaningful, is that the UUA is not meeting the needs of all Unitarians in the US. Certainly, it is meeting some needs, and enough needs to warrant its continued existence. And surely, their may well be many self-identified UUs who have no need for an such an organization. But between these two groups, we have a bit of a diaspora. It's a bit like the situation with YRUU, actually.
The trick then is to determine whether the current institutional structure is serving the needs of those who belong to it, to consider changes that can be made to reach out to those who are hesitant, and to acknowledge that one institution cannot be all things to all people. YRUU is often the first time that youths, en masse, re-prioritize their time away from Sunday morning RE to other commitments. It's an entirely appropriate time for a re-evaluation. Duncan, in the comments at YRUU UUlogy, says
Part of the conclusion that many people have reached (read: Ministers/staff) is that the entire MODEL of the youth group as it is largely practiced is not sufficient and should be scrapped. They believe that cons encourage youth to only attend cons and youth stop going to their churches. They think that too many youth leave after Coming Of Age, and they believe it is because YRUU groups don't meet a wide enough set of needs.
I think in my experience as an advisor a lot of youth leave because A)they had a deal with there folks that after CoA they are not forced to go B) other demands such as theatre or sports C) with other demands sometimes Sunday mornings are the only one in which people can sleep in. And not about the youth group.
And this is the second point I want to make here; this is why I opened with the converts experience. As the Coming of Age teachers at FUCA are fond of saying, "at the very heart of our seven principles is 'A free and responsible search for truth and meaning'". I cannot posit that all of the self-identified UU diaspora are the non-attending children of Unitarians, but I can safely say that some of it is. Because church, even an open and free church, is not for everybody all the time.
And that's the third part. We have in this nation the "Immigrant Experience" as a cultural narrative, and it is increasingly the dominant one. Not immigrants as in pilgrims, but as immigrants after the US was founded, who entered this country knowing what it was, without having been present for the formation. It's an interesting narrative, and a valid one, but it has complications of it's own, creating dissonance between those who acculturated into things like white privilege and those who can trace their heritage back to the very beginning when things like privilege were codified into law. This is not to say the UUism has nativist sentiment or a burden of guilt, but that sacred narrative, that powerful experience of how much better this is than the state we willingly left runs into trouble when youths, raised in the church and without the experience of conversion, find flaws in their parent's land of opportunity.
This is another narrative, the dual identity of our nation, our faith, and our congregations, and it's a complicated muddle. YRUU is only a part of the experience, but I think it's an important piece of UU identity, at least among convert teens and 2nd (or later) generations of UU youth. And odds are, a good many of us will take our free and responsible search for church and meaning within the denomination, provided that's allowed.