Every website is an island. Oh, sure, they can hyperlink together, and some of them are designed to make use of another website or maybe a mobile or desktop app, and some of the really far-thinking ones provide an API you can access, but essentially every website is disconnected from every other one. If you have data here, but you want to use it over there, it's a pretty big deal if a website actually offers you a way to do that. Contrast with the desktop software world, where my text files can be opened by practically any program I want, I can copy them anywhere I want to, I can cut them up and use them for different purposes and all kinds of things.
This is why integrating websites and web services is the new secret sauce. Just about everything you need or want is out there somewhere already, even if you want website A, but with just a bit of the feature from website B. Unfortunately for you, combining the two is only possible by (a) writing a third web service to use both their APIs in concert, if such things exist, or (b) writing a third web service that duplicates all the same desirable features as the first two.
That's what "web 2.0" is/was supposed to be about: web services becoming programmable interfaces so that we don't need to reinvent everything every time. It's the old Unix philosophy where each tool does one little thing really well, and if you need something new, you probably just need to string a few existing tools together in a new way. Despite the hype dying down about whatever we thought "web 2.0" meant at the time, I think it is the way forward. We're just starting to say "cloud" now, and meaning the same thing.
Mokalus of Borg
PS - The internet itself is the development platform.
PPS - But it also needs to include the existing websites and tools as building blocks.