Well I found a site which says there were about 16 million Jewish people before WWII. The world population in 1930 was 2 billion. In 1940 it was 2.3 billion. So let's take 16 million / 2 billion = .8%. Now in modern times I believe it's .2% across the world (2% in the US).
Does anybody see other figures for either of those two numbers?
From what I've read it seems that the Jewish population has stayed pretty steady - but since the world population has grown, that has caused the percentage to drop relative to the growing world population. Starting from post-WWII of course.