If you're a tweeter, as I am (but just barely), then you've seen the Twitter banner page. If you're an anal-retentive grammar nerd, as I definitely am, then you've been bothered by said banner page.
Who to follow. This is wrong. Who, in this case, should be Whom. It is the object of the verb follow.
Who is correctly used only in the subjective form, that is, when it's the subject of the sentence, e.g., Who is the best one to follow? Who is the subject of the sentence, similar to he: He is the best one to follow. (No, I'm not being sexist; of course she also works, but I'm using he for a reason, which you'll soon see.)
In the objective form, when it's the object of a preposition or verb, who becomes whom: Whom should I follow? Follow him. (Notice how they both end in "m" -- that's the trick I use, and that's why I'm using only the male pronoun here.) You wouldn't say Follow he, so you similarly shouldn't say Follow who? or Who should I follow?. (You could also turn the sentence around to figure out which is right: I should follow him (whom).)
Now, if you're my father-in-law, you simply bellow "WHOM!" whenever you hear who used incorrectly. And you charge a nickel for each offense. Generous person that I am, I'm sparing you -- and Twitter -- that expense.