This
Pew Research study of religions in America is getting good exposure. Haaretz broke down some interesting stats
here. Here are some things I took away:
1. Only 1.7% of America is Jewish (Comparable to Mormons)
2. < 0.3% is Jewish Orthodox. So there are at most ~900,000 (age 18+) of "us" in this great country. Pew's methodology states that they conducted random telephone interviews. I would guess that the "ultra-orthodox" a) have cell phones and not land lines b) are less inclined to answer surveys in general, and may be unable to do so because of language barriers.
3. 72% of Jews (18+) have no children. 82% of Conservative Jews have no children, the highest of any religious group (compare that to 46% of Jews making $100,000+, the highest for any religious group). Even if we consider that the 18-30 demographic is not as likely to have children, that still leaves 57.6% of Jews 30+ and 64% of Conservative Jews with no children. As a further piece of datum that I do not know what to do with, 81% of Jews have been in a committed relationship at some point in their life. How much overlap there is between the remaining 19% and the 18-30 age demographic is unclear.
Let's do a sad back-of-the-envelope calculation. Let's assume that 55% of Jews marry non-Jews and of the 45% of remaining Jews 58% of them have no children. Let's also guess that the avg # of children for those that do decide to have children is 2 (which is about right). In that case, every 100 Jewish adults produces 19 Jewish children! That is a total low ball figure, as ~50% of those that inter-marry still produce halachikally Jewish children, but there is a rather high chance that they will not affiliate as well.
19 for every hundred is not replacement, no where close.