I think you're largely right, but IMO the difference between what happens online and what happens offline is that you have to look people in the eye offline and you may encounter them randomly in person. There's a moderating function that generally (not always) keeps the heat down.

Same for pile-ons. I'm not so naive to think that they never happen offline, but there's a higher cost to doing them than there is when all you have to do is write 140 characters to dunk on someone you won't even remember in two weeks.