I think PE ratio is something that can not stay high forever. At some point it has gotta catch up with the profits. I can understand that Amazon reinvests most of its profits and can "at-will" switch itself on to generate profits but the problem is that this future switching-on is already been included in their current PE ratio for most part. So the real issue is that it's damn hard to calculate the right price of stock with correct margin-of-sefety.