You might call it a social backend. Imagine Twitter+Facebook+Dropbox but with all the UIs removed, so it's just a backend datastore with APIs. Then third parties write apps that provide different UIs on that data.
But isn't Twitter without a UI or userbase just basically a database, only controlled by a third party? What's the point of that? I already have MySQL and Postgres, and they are free and open source.
It's a massive social graph + infrastructure for subscribing to data sources (i.e. other users) and broadcasting messages from one source to all subscribers (including push).
And that's just Twitter. ADN includes a lot more than Twitter does.
