This thread got me curious, so I went Googling.
Here were some of my findings:
Statistics for JWs did have them the fastest growing denomination in the U.S. in 2008.
GLTirebiter made a good point that the numbers reported in the NCC Report are numbers reported by the denominations, not an independent survey. An ABC news analysis of that report for 2011 (which was numbers from 2009 reported in 2010) said the figure for Jehovah's Witnesses was misleading because they only report "active" members rather than attendance. They said a more accurate indication growth would be a comparison of attendees to their annual Memorial observance.
Also, the report stated that 10 of the 25 top denominations (of which JWs are listed as 20th) do not report.
So the report appears to be numbers from 2009-2010.
The latest reports from 2011 figures say that the fastest growing denominationin the U.S. is LDS, followed by Islam. (Some reports have it the other way around.) And the fastest Christian religion in the U.S. is "non-denominational" churches, away from mainstream religions.
Here's the link:
http://www.standard.net/stories/2012/05/04/religion-census-lds-church-fastest-growing-group-us
All in all, from this rudimentary Google search, it appeared that JWs were the fastest growing denomination in the world in the mid-1960s, faring well in 2008, but nothing apparent for 2011 to the present. The Pew Forum on Religion & Public Life did report the numbers from the NCC report, but also stated in the most recent report that JWs have the lowest retention rate of all groups.
I guess people can find stats to support any agenda they want to support.
Cedars: Thanks for your comment above. Just thought I might be confused. :-)
~Binadub