The most important sentence of @edd Google+ is the social backbone is:
[Google is] a company for whom exclusive ownership of the social graph isn’t essential to their business
That’s the crucial point I made before that would enable Google+ to work in a federation, like email.
Imagine having the possibility to run your own Plus-like service and still be able to comment on your brother’s pictures, hosted on his own server. Google is the only company that can do this and still profit from it.