Hat tip to volokh.com for a post that links to a Brandeis’ University study on Jewish population in the US. I’m not much interested in the science of demographics and how exactly you count heads, but some of the results are interesting. The Brandeis study claims that earlier studies overestimated the # of Orthodox Jews and overestimated the # of Jews receiving a day school education, and is critical of the NJPS survey of 2000 for undercounting the non-Orthodox. One interesting statistic (footnote 21 on p. 37) is the claim that over 80% of those who identify as Orthodox Jews were born and raised into Orthodox families. The survey considers this a lack of evidence of an upsurge in Orthodoxy. I guess that conclusion depends on your perspective, because to me the statistic that 1 out of every 4 Orthodox Jews are ba’alei tshuvah who chose to return to or embrace Orthodoxy without being raised in an Orthodox home is a very significant number. Yes, far larger numbers of Jews raised in Conservative homes switch to identifying as Reform Jews, but that simply reflects a loosening of commitment, not a choice of Reform philosophy over that of the Conservative movement; the choice to become Orthodox is clearly a commitment to greater religious practice and observance. Also interesting is the finding that esp. among Orthodox, there is a population bulge among 18-29 year olds, 53% of whom are married (shidduch crises?) compared with only 18% among non-Orthodox peers (who they claim have been undercounted). The conclusion I draw is that the Orthodox population is younger, larger, and bound to continue to grow (marrying younger increases your odds of being able to have a large family) faster than other segments of the Jewish world. I haven't read the full study too carefully, so have a look if you want more detail.
Update: After posting, I noticed this review of the survey from kumah.org. They are a bit harsh, but many of the criticisms strike me as valid.