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?