Who was late? (Who is the subject of the sentence.)
She was late. (She is the subject of the sentence.)
I did not know who arrived late. (I is the subject of the sentence and the subject of the main/independent clause, while who is the subject of the noun clause/subordinate clause/dependent clause.)
Nominative pronouns are also used for predicate nominatives, but these tend to confuse people, so I'll save them for another post.
But whom is objective case, as stated above. This means it is used as the object, usually the object of a preposition.
For whom did you make this cake?
Ask not for whom the bell tolls.
To whom did you send the e-mail?
By whom was this portrait painted?
(All these sentences have whom as the object of a preposition.)
Simply put, then, for those of you who didn't major in English, if the pronoun you want is the object of a preposition (to, for, by, near, across, with, between, etc.), use whom, not who.
Thus, this ad for authors is, rather ironically, dead wrong, as far as grammar is concerned.
And Twitter's suggestions of people I should follow
Who to follow· · View all
are always grammatically incorrect.
(The correct versions should read:
For whom am I writing?
Whom to follow)
Now, if you feel that sentences using whom sound too formal, you can always reword.
People you might want to follow:
Authors, who is your intended audience?
But poor grammar can make a writer look ignorant -- or just plain stupid. Take care to know the rules, even if you intend to break them for stylistic effect at some point.
(Note: whoever and whomever follow the same rules as who and whom, respectively, but whomever is often the object of a verb, infinitive, etc.)