It's more than a platform (OnPlatforms) because it's not owned by anyone.
Compared to what we're used to, this rapid proliferation of easily accessible APIs is so, uh, so uncomputer-like. Whether or not those new APIs get used is a different story. But the point is that there's no roundtable of Jedi Knights through which all proposed kernel changes must pass. And, much the same way new mashups keep showing up every day, so too, as TechCrunch editor Mike Arrington constantly reports, do the APIs (Arrington is demonstrating a knack for getting the scoop on new APIs, blogging about them almost as soon as they become available).
Still not convinced of the uncomputer? Well, then consider this: not only is anybody free to add a new API at anytime, the primary user interface a browser almost never needs updating to take advantage of those new APIs. Pretty uncomputer-like. Compare that to what happens when a classic operating system takes on new APIs. The upgrade cycle can be incredibly painful, requiring all sorts of special hardware, new software and budget exercises that, years from now, when millions of mashed-up applications are available to anybody regardless of what technology they have in front of them there will be a lot of people looking back at the old way of doing things saying "What in the world were we thinking? Why didn't we do this sooner?"