“By whom?” vs. “Who by?” [duplicate]

Possible Duplicate:
What's the rule for using “who” or “whom”?
With who vs. With whom

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?

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?")

"By whom?" is correct. "Who by?" is incorrect, though it is commonly used, especially in speech as opposed to writing. Actually I suspect the most commonly used expression would be "By who?" "Who by?" is really doubly incorrect because it both uses the wrong form of the word "who" and it puts the object of the preposition before the preposition.

To those who say that correctness is determined by popular usage and that "whom" is becoming obsoleete: Yes, but usage by whom? (Or "Usage who by?" if you prefer.) "Whom" is still considered the more literate and correct word to use. If you want to sound intelligent, write "by whom". If you're chatting with your buddies at the bar, it doesn't matter.

Whom and who in object position or in a prepositional phrase are both grammatical in Standard English. The difference is one of formality, with whom being used in the most formal contexts. ‘The Cambridge Grammar of the English Language’ says all that needs to be said on the matter on pages 7 and 8 here.

I've heard of "who by" for the first time! It is, naturally, grammatically incorrect. However, like @speedyGonzales said, it is okay to use it for spoken English. But I've only heard of "by who".