word choice - “By whom?” vs. “Who by?”

Are “by whom?” and “who by?” perfect equivalents?

I have the feeling that the use of “who by?” is just a way of avoiding use “whom” but I have no evidence or proof. The anglophone people I talk to hardly use “whom”.

Are both expressions equally used, or is the difference between them only regional?

Answer

They're not perfect equivalents, but they're pretty close. The biggest difference, of course, is that whom is a pretty formal word, so "By whom?" is a very awkward reply to, say, "He got f'd." Either "By who?" or "Who by?" would be much more natural.

Another difference is that if someone uses a by-phrase that you didn't quite catch, or that you're surprised about, you can reply "By whom?" or "By who?", but not "Who by?", to request a repetition. ("By who?" is the most common wording in this case: "This book is by Mr. Aasefalsdfjaose." "By who?")