I thought this may be of interest to some, don't know if its been posted before.
AMERICAN RELIGIOUS IDENTIFICATION SURVEY, 2001
THE GRADUATE CENTER OF THE CITY UNIVERSITY OF NEW YORK
http://www.gc.cuny.edu/faculty/research_briefs/aris.pdf
It is 47 or so pages long, so I didn't embed it.
On page 13 the JWs are listed and it shows decrease from 1990.
I don't know how acurate the methodology of this study was, wish I could borrow Leolaia's brain.
I thought it was interesting to note what religions are decreasing compared to increaseing.