The American Religious Identification Survey (ARIS) study is an independently funded study that asks a sample of the people in the US, "What is your religion, if any?" It has occurred three times: in 1990, 2001, and 2008. In 2008, 54,461 telephone surveys were conducted that asked this question.
For more information on the survey, see: http://www.americanreligionsurvey-aris.org/
What you won't find on that site is information that has been previously published on New Religious Movements (NRMs). That category, as you will see in the Appendix on the page above, includes Scientology. The reason is that NRMs tend to be small individually. The results for the smaller religions have greater error due to the sampling methodology. That is, they tend to be imprecise and unreliable due to the sample size. Thus, the folks who run the ARIS study aggregate the data for the NRMs (Wicca, Druid, Eckanar, etc.) in an attempt to bring some broad stroke understanding to what is going on with respect to religion in the US, and to minimize possible media misinterpretation of the data.
Still, I think the disaggregated data is of interest, and they have published it for the 1990 and 2001 surveys. Thanks to a direct inquiry I made to the research team at ARIS, I was provided with the disaggregated (albeit summary) data for Scientology membership in the 2008 study. PLEASE see my comments after the data before drawing any conclusions; treat these fluctuations with caution:
Self-identified Scientology membership:
My comment: This may be the first public release of the 2008 data for Scientology. Should you conclude from this data that Scientology membership has more than halved between 2001 and 2008? NO! Again, there is a significant chance of error (reported on one site as possibly as much as +/- 40%) in the results for religions of such small memberships (by way of scale, the leader, Catholic, came in at 57.2 million). Other factors may have been involved when the study was conducted: perhaps when Cruise was at his, er, nuttiness, members were adverse to identifying themselves as Scientologists.
Still, I think it is correct to conclude that a) Scientology has certainly not grown in the US since 2001, and b) it likely has gotten smaller in the US since 2001 (at the same time as the internet/media info on the COS grew exponentially beyond ars, etc.). The claim that there are 10 million Scientologists worldwide remains, as usual, safe from any rational belief that it is anywhere near the truth. Indeed, it could be overstated by as much as two orders of magnitude. (100 times overstated!)
I will try to forward this info to Hartley Patterson (who has a website on this topic).