I'm a big user of and believer in AWS. It's got lots of components: databases, storage, computing, and lots of things with fanciful names that I don't really understand.
I've been using AWS since its inception. Their first service, S3, provided something you couldn't get anywhere else without buying lots of other stuff -- storage. And the service was:
S3 set the pattern for all the subsequent AWS services. And they're delivered so many, filling almost all the niches you could imagine, sometimes with multiple products. But the one niche they have never attempted to fill is what Twitter does. Real-time Internet-scale notification with an easy to understand user interface. Turns out this is one of the big things that was missing from the Internet itself.
It took a long time for Twitter to get their servers to run reliably (the first two items on the list above). But they've never offered it as a service. In fact, they pulled back from the API, which was heading in that direction. It would have been a very different world now if instead they had gone in the other direction, by turning it into a web service for any and all developers to build on any way they want, the same way AWS does, for a price.
Because there is no web service that does what Twitter does, yet -- it's not too late for Twitter to open up another business model. I think it would totally kick ass. We need it. And I think they'd quickly forget that Twitter was ever going to be, exclusively, an advertising-based system.
And of course they could continue to operate that system, as Amazon continues to operate their shopping and delivery business, even though they see that as a proving ground for the web services, which believe it or not is their real business!
Or one more possibility, if Twitter doesn't want to do this, would Amazon please do it! The world needs Twitter-as-a-service. We've waited way too long for it.