Well, Apple, Google, Mozilla and MS control the browser scene, and that's what I think Joe meant by platform. Things are a little more complicated in the backend.

You could add Facebook to the pack but, though it has changed how people use the Web, it didn't contribute much to improving the platform.

And in a sense, although it may sound idealistically foolish, all of us. A long time ago, I helped fixing a bug that prevented Webkit and Gecko from connecting to my bank's crappy site, making me launch VirtualPC just about everyday. A few years later, I wrapped a perl script in a GUI that spared the users of a broadband provider from having to manually login in on the carrier's “portal” just to use the Web. A lot of people used it and they eventually changed that annoying behavior. There's nothing stopping you from trying to have an influence.