Amazon's profits 
Of all the big internet giants, I think Amazon will be the hardest to disrupt.<p>Androind could eat away at Apple.  Google's search could in theory be disrupted.  Facebook seems more vulnerable by the day.  Microsoft is a dead man walking.<p>But Amazon has this entire subsystem of distribution centers filled with inventory risk.  People focus on the website, but I think the real magic is everything that happens behind the scenes.  The barriers to entry are enormous.<p>I once created a comparison shopping service.  When we talked with users, we heard over and over, "I just go to Amazon.  I trust them.  I don't shop around online."   Or, "I have Prime, so I'll save money on shipping anyway."  It made the entire space seem pointless.<p>An internal Amazon company motto is, "it's still day 1."  That's their view -- that it's still the early days of the internet.  That's why they reinvest so heavily, because it's still early days.