That being said, a big part of me wonders if (assuming you could find lawmakers with enough technical literacy to propose it, which would be a pretty huge ask!) you might be able to get the best of both worlds by requiring sites like facebook to expose an API for programmatic access. That would solve the stangancy problems that Moxie highlights (not to mention sidestepping the need to get everyone to agree on any one protocol), whilst simultaneously forcing a whole lot of things to open up.
Alternatively: part of the reason these walled gardens exist is that they control the concept of identity. Not necessarily in the sense of "this is my profile", but more in the sense of authentication and authorization. If you moved that somewhere else -- say, in the browser -- it would make it much easier to build systems that are interoperable.