I wouldn't compare OS interfaces with websites. They are totally different things.

An OS UI is mostly buttons and other widgets, window frames, and behavior. The actual applications are very different.

Just to give you an example:

Look Reeder app, Chrome, Things, iPhoto all for OSX. They are wrapped with the a common UI, but they are much different from each other.

Sure they use lots of the same OS widgets, window frame,etc.. but they have their own design and way behaviors inside.

With frameworks like twitter you can't get that, unless you hack the source. But that defeats the whole point of it in the first place.