Jews are not losing many of their own: One perennial concern in the Jewish community is that people are marrying non-Jews, leaving the faith, and are not raising their children to be Jewish. The GSS has some relevant data here. First, I looked at the children of marriages with either one or two Jewish parents. Here is the breakdown:

28 total children

25 had two Jewish parents and were still Jews as adults
1 had a Jewish mother and a Protestant father and grew up to be Jewish
1 had two Jewish parents and as an adult became a Protestant
1 had two Jewish parents and as an adult had no religion

Another question asks adults (in another sample) if they ever switched religions. For Jews, the numbers look like this:

10.1% switched to no religion
3.0% switched to Protestantism
1.6% switched to Catholicism
0.3% switched to Buddhism
0.9% switched to some other religion


As for intermarriage, I included all cases that involved any Jewish membership. In 26 cases, the person stayed Jewish:

20 people were raised Jewish, married a Jew, and remained Jewish
2 people were raised Jewish, married Catholics, but remained Jewish
1 person was raised as a Jew, married a Protestant, but remained Jewish
1 person was raised as a Jew, married someone from some other religion, but remained Jewish
2 people were raised Jewish, married someone with no religion, but remained Jewish


In three cases, a person switched from their Jewish upbringing:

1 person was raised a Jew, married a Protestant, but became a Buddhist
1 person was raised a Jew, married a Jew, but became Protestant
1 person was raised a Jew, married someone with no religion, and had no religion himself


But in 2 cases, non-Jews became Jewish:

1 person was raised Protestant, married a Jew, and became a Jew
1 person was raised with no religion, married a Jew, and became a Jew


And 10 others married Jews but did not become Jewish:

2 people were raised Protestant, married Jews, but remained Protestants
1 person was raised Protestant, married a Jew, and as an adult had no religion
4 people were raised Catholic, married a Jew, but remained Catholics
3 people were raised Catholic, married a Jew, and as an adult had no religion

So on the question of losing Jews, only 3 in this sample left the Jewish faith. Two of the 3 did not join the religion of their spouses, and the third switched to no religion (which was the same as the spouse). So three Jews were lost, but two were gained through intermarriage to Jews. I don't see much evidence that intermarriage is making Jews disappear. A very small percentage of Jews are switching to Christianity or Buddhism--more are lost to irreligion, and I imagine that many of these folks consider themselves to be Jewish in an ethnic sense. And as the top set of numbers show, 26 out of 28 children with at least one Jewish parent grew up to be Jews.