As far as I can tell, 100% of Unitarian Universalists will, in time, stop being Unitarian Universalists. Death does that.
Whether we experience the Beatific Vision, are reincarnated, subsume into the Monad or simple stop being we share a common experience after death. I've never heard anyone suggest we will carry on as a sect (and I hope I never do.)
But before death, many people who have at some time identified as Unitarian Universalists no longer do so. Another Unitarian Universalist Christian minister I know has found a path in ministry in a Christian communion; this has happened before and at one point I thought this would happen to me. Even now, Mama G (Mom to the Left) has been considering leaving her Unitarian Universalist church. This happens a lot, and as the Pew study talked about today shows, happens in many religious traditions. But I suspect it hurts a bit more when you feel small and marginal. I know Universalists wrung their hands about making the numbers work well more than a hundred years ago.
But I don't think the question "how many Unitarian Universalists are there?" is any more helpful now than then. Some people are very engaged and others aren't. Some hold on to their church affilations as if life depends on them -- yet are not members -- while some lifetime members regard their status without a second thought. Numbers only tell part of the story and I suspect it isn't the most important part.
It follows that I'm not particularly concerned about how many Unitarian Universalists are "missing"? (I do think that good record-keeping is important within a congregation, for some benchmarking, and for resource allocation across the Association.) Why? Each of us only has so much time in this life and each of us shares so much ministry with others and the world.
I am concerned we get caught up in little dramas and dubious joint projects that would never pass in well-functioning secular organizations.
I am worried that, while divergence of opinion is becoming a recognized public virtue, Unitarian Universalists experience pressure to develop singular expressions of identity.
And I think that counts for something.