The websites google.com and facebook.com and twitter.com (to distinguish from their mobile apps) are very popular, but the most part these sites just render text and images.

google.com doesn't try to search my local drive. faceboook.com doesn't offer to share my local photos. Their usage of "browser as an OS" is extremely limited.

I worry about the other problem: the browser becomes so ubiquitous the OS disappears, along with any platform conventions and application interop.