"Who" is a subjective pronoun, and "whom" is an objective pronoun.  Whether you should use "who" or "whom" all depends on Case, which, aside from what beer comes in, determines which pronoun you should use depending on how you are using it.  If it is the subject of the sentence, it should be "who."  If it is the object, it should be "whom."  (Who's there?  Whom are you with?)  But here's the thing:  Aside from somebody trying to prove they paid attention in college, who or whom cares?  The problem with “whom” is that not many people get it consistently right to begin with, and nobody really cares if you don’t, except maybe for those people who get it consistently right.  And you’re probably not hanging out with them anyway.