Just like every other "ism" I can think of, from Feminism to Objectivism (100%, both), Socialism was, from the start, populated by a wildly disproportionate number of nonreligious Jews.
Indeed, "Marx" is a Jewish name, and I'm pretty sure Carl's grandfather was a Rabbi from a prominent family. (Don't have time to check right now.) The Socialists borrowed greatly from the Communists, no? Truth is, I think a classic Israeli Kibbutz would fit very nicely into either "ism". They even had communal child raising and meals, with both parents working.
Considering that less than 1/2 of 1% of the world's population (<20M/>4B) are Jews, a HUGE number of the early Socialists were. (Outside of England, some parts of Europe thought of Socialism as a Jewish thing.) Indeed, the typical European Jewish community was composed of 3 or 4 large groups: Religious (Chasidim) and/or Religious (not Chasidim), Socialist, and Zionist. In many towns, the Socialists outnumbered the Zionists.
Indeed, most of the early emmigrants from Europe (when they could still leave) were Socialists, and not really Zionists. The joke in Miami (where you can still find enough survivors to joke about it) is that the Zionists mostly perished in the "camps," because they stayed behind encouraging everyone else to go to the Holy Land, so the U.S. and Israel now have mostly the decendants of the Socialists. This, like most Jewish humor, is pretty much true. Most of the religious community perished, as well, since they didn't listen to the Zionists, and only a very small number of Rabbis and heads of Yeshivas told people to leave.
Which ones are very easy to tell today: just look for populous groups of Chasidim and European Yeshivas that are now open in the U.S. or Israel: Satmar, Bells, Lubavitch chasidim, and the Mir and Ponevitch yeshivas. That's about all that really survived in significant numbers, out of many dozens of groups of chasidim and dozens of yeshivas. A tiny percentage of everyone else survived the "camps." A few, like the Square Chasidim, managed to rebuild in the U.S. or Israel from the tiny number of survivors.
There are lots of theories as to why so few religious figures told their followers to leave while they still could, but none that I like well enough to repeat.
Some theorize (see my India comment) that a Jew, when not well educated in his or her own religion, has a yearning that they will inevitably find SOMETHING to fill. I'm partial to this theory, myself, as I'm a scientist at heart, and it explains a lot.
According to this theory, whichever "world saving ism" is ascendant at the time will attract a lot of nonreligious Jews, and that seems to have been pretty much the case, for the past 150 years or so anyway.
[Edited to fix bad typing]
