I’d bet very small numbers are fluent still around the world. As for using little bits of Yiddish here and there, yes, it’s very commonplace among Ashkenazi Jews, and many Yiddish words have been adopted into the English language in America, especially the urban dictionary. In certain industries even many of the non-Jews use the terminology regularly like the garment industry, Hollywood and theatre, and more. You probably use or know some Yiddish without realizing it. I have noticed more use of it on TV lately. Just the other day Chris Matthews used mishigas, and I thought to myself a lot of people won’t know what it means. Schmuck, schtick, yutz, putz, schnook, mashugana, nosh, schmear, I could go on and on. All commonly used in many parts of America by Jews and people who aren’t Jewish.