A description of what is and is not an instance of the ad hominem argument. Apparently this has been around for a while and has been discussed at length. I’m not sure all the examples the author cites as ad hominems are ad hominems (for example, B’s reply, “Yet another ad hominem argument. Ignore this one, folks.” isn’t really attacking A but is attempting to divert the attention from the actual argument A is making and so might be more of a straw man argument (or perhaps an ignoratio elenchi)). But in general, the author’s point is a good one. An ad hominem ignores the actual argument being made and attempts to undermine the conclusion of the argument by attacking the person.
Thanks to Andrew Smith for the pointer.