A continuing debate in English usage is the question of when to use "who" and when to use "whom."
According to formal grammar, "who" forms the subjective case and so should be used in subject position in a sentence, as in: who decided this?
The form "whom," on the other hand, forms the objective case and so should be used in object position in a sentence, as in: whom do you think we should support?; to whom do you wish to speak?
Although there are some speakers who still use who and whom according to the rules of formal grammar as stated here, there are many more who rarely use whom at all; its use has retreated steadily and is now largely restricted to formal contexts. The normal practice in modern English is to use who instead of whom (and, where applicable, to put the preposition at the end of the sentence): who do you wish to speak to?; who do you think we should support? Such uses are today broadly accepted in standard English

Credit- Azuka Onwuka
-- oxforddictionaries.com