Careful speakers and writers--particularly older people such as me--still use "whom." However, it is becoming outdated. Language evolves. The tradition in English is for dictionaries to record the language as it is, not as what it "should" be. In the US, sounding too educated is perceived as "showing off" one's social status. In the US, there is a regular evolution, in which older "correct" usage becomes too formal, and is gradually replaced by English as spoken by middle-class speakers who are not "uneducated" but not "highly educated." I really like the American Heritage Dictionary, available online at no cost. One of the things I like about it are its usage notes, which I think are excellent. Instead of being one person's opinion, they summarize the opinions of their "usage panel." They have one here: https://ahdictionary.com/word/search.html?q=who This is just a part of what they have to say. First, they outline the details of traditional correct usage. Then they go on to say: "Despite the traditional grammatical distinctions outlined above, in practice 'whom' is uncommon in speech and everyday writing because it has a formal tone. In informal contexts, 'who' often replaces 'whom,' as in 'Who does the actor support?' or 'I despise the governor who the actor supports.' A common workaround for the problematic choice between formal 'whom' and grammatically questionable 'who' is to replace the relative pronoun with 'that,' converting 'the governor whom the actor supports' into 'the governor that the actor supports,' or to omit it altogether, yielding 'the governor the actor supports.' 'Whom' survives as the standard form when it is the grammatical object of a preposition that immediately precedes it, as in 'the governor for whom' (not 'for who') 'the actor campaigned.'"

So technically speaking, the following rules are true: - "Who" should be used to refer to the subject of a sentence. ---> "Who is your best friend?" - Whom should be used to refer to the object of a verb or preposition. ---> "Whom do you believe?" However, "whom" is considered to be very old fashioned and formal nowadays, so people find it more than acceptable to just always use "who" instead.

Dan has given an excellent answer. To add to his comments, I think the reason why "whom" is disappearing is because it's an unusual word. English doesn't have a case system that distinguishes between nominative, accusative, dative, etc. -- except with a few pronouns, like "I/me" and "he/him." So English speakers are not used to thinking about cases. In fact, English speakers even have problems using the correct case for personal pronouns, which is why they often say things like "Me and John are leaving" or "It's a gift from Mary and I." English speakers don't understand cases. They know that you use "I" before a verb, and "me" after a preposition, but if they have to use a pronoun in an unusual place in a sentence, they're not sure what the rule is. Because people aren't used to changing words depending on their case, they tend to just always say "who" instead of worrying about whether they should say "whom." And as Dan says, this is now becoming the standard, since it's so common. Highly educated speakers do know when to use "whom" correctly, and in writing, they usually do use it. People who have studied foreign languages that have case systems, like Russian, usually have a good understanding of the use of cases in English. You'll certainly still see the word "whom" if you read professional books. Even uneducated speakers generally recognize that it sounds wrong to use "who" after a preposition like "to" or "for," but since they don't want to use the formal word "whom," they'll just rephrase their sentence. Educated speakers sometimes have a similar problem, and will rephrase a sentence to avoid using "whom" because they don't want to say "who" (which is grammatically wrong), but they also don't want to use "whom," which tends to sound pretentious.

Whom takes a preposition (to whom / by whom) etc.

Using “Whom” is just a little old way of speaking, but still can be correct. If you wish to use it then “Whom does he like?” is the correct way! :)