Amazon's profits 
Of all the big internet giants, I think Amazon will be the hardest to disrupt.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.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.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.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.