Do people use whom any longer? Many teachers have told their classes that the word whom is no longer used, so they will not teach it. They claim that it is out of use like thou and ye. Instead they instruct their class to use the word who instead of whom. The word whom is the objective form of who (link.mingpao.com/67107.htm).
Are those teachers correct? Has this word died out? Well a quick check of the Corpus of Contemporary American English shows that to who does get used often enough in American English to be acceptable, but only one-third of the time. Two-thirds of the time to whom is used. So, while not wrong, using who for the object of a sentence is not the preferred option. According to the British National Corpus in British English, to who only gets used one-eighth of the time, instead of to whom. The word whom is clearly still in use.
Checking for other variations in use, also shows that whom is preferred as the object of a sentence in American English and strongly preferred in British English. In American English especially, the word who is accepted as an informal substitute for whom.
The phrase To whom it may concern is very often used to start a business letter when one does not know the name of the person to whom the letter is being sent. This is a fixed phrase and will likely remain so, even if other uses of whom are abandoned.
If one uses whom as the subject of a sentence, it is definitely an error. Furthermore making such an error, when trying to be very correct is called hypercorrection and can sound silly.
My advice to students is that if you are sure that the word is the object of your sentence, use whom. So never use to who, but only to whom. If you are in doubt use who; it is still acceptable in American English. The word whom is alive and well in British English and is still often used in American English. So don't tell your students that the word whom is no longer used.