FlickrGraph wouldn’t at all be possible without the Flickr API, which exposes many of the functionalities in Flickr as REST, SOAP or XML-RPC calls to any developer interested in using them.
This is what Jeff Bezos points out in his Web 2.0 talk:
Web 2.0, from where I stand, is about making the internet useful for computers.
Blogs with RDF/RSS/Atom-based aggregation were only a very tiny first step to making content and services more widely available, and now everytime I think of a useful service on the web, I end up sketching out its API first, figure out a few interesting uses for it, and then decide if it’s worth or not implementing.
If you look at Flickr’s insane popularity rise, there are definitely a few lessons to learn about a great web application, and providing an API is certainly one of them. It’s an incredibly dense app, even with the apparent simplicity, so I’m sure there’s a lot more.