Twitter has a "who to follow" button on the upper side of the screen. Shouldn't it be "whom to follow"?
Wikihow suggests that whom is the correct usage in a case like this.
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Let's start by taking the truncated phrase and expanding it to a longer more recognisable sentence, while comparing it with another example of the same structure.
What to do - what is one to do? - what should one do? - what should I do?
Whom to follow - whom is one to follow? - whom should one follow? - whom should I follow?
Here, the answer would be 'you should follow her/him', which means that the 'whom' in the question is referring to the object in the answer.
That makes 'whom to follow' correct, since 'whom' should be used in objective cases and 'who' in subjective.
(since the title 'whom to follow' is not given a question mark - it could also be expanded thus, though it is a little more complicated than the example I gave at first:
What to do. - what one is to do - what one should do - this is what one should do - one should do this - you should do this
Whom to follow. - whom one is to follow - whom one should follow - he is a person whom one should follow - one should follow him - you should follow him
again, the expansion resolves itself with 'whom' referring to the object.)
"Whom", where it is used at all, generally belongs to a somewhat formal register, particularly when it is not used after a preposition. (I would go even further and argue that it's basically an artificial invention and not even part of "natural" English.)
So one might actually argue that on Twitter, it shouldn't be "Whom to follow", because that would be of an inappropriately formal register for the context
 Just to clarify in view of comments below: once upon a time, English did indeed genuinely have an overt case system and any native speaker would have naturally acquired and systematically used that overt case system, just as happens in German, Dutch, Russian etc today (and just as in those languages today, there were surely some speaker-to-speaker variations 'around the edges' in precisely what case form occurred where, but as a system it was stable). But that's essentially irrelevant. As the language stands to today, I suspect that the who/whom distinction is an artificial invention.