Seeing that Twitter has 500 million users, an order of magnitude more would be in the range of 5 billion, and a second order of magnitude would be around 50 billion.
So I seriously doubt Amazon has "orders of magnitude more users" that Twitter. For Tumblr, maybe, but I doubt that too (maybe just one order of magnitude for it).
Besides, all that's orthogonal to my point. Except if you mean that the reason that Twitter and Tumblr have less users than Amazon is that they left those services due to outages.
Twitter has much better uptime than it used to, too.
When you get in the hundreds of millions of users, you've run out of early adopters and end up with users who are a little more demanding about uptime.
So I seriously doubt Amazon has "orders of magnitude more users" that Twitter. For Tumblr, maybe, but I doubt that too (maybe just one order of magnitude for it).
Besides, all that's orthogonal to my point. Except if you mean that the reason that Twitter and Tumblr have less users than Amazon is that they left those services due to outages.