What happens every time this is proposed is that the largest networks never join, since the last thing they want is to hand their best competitive advantage over to the competition. And then the project dies.

I think the bigger problem is maintaining the data. The advantage Facebook has is that your profile is always kept up-to-date implicitly by the act of you using it. You "like" something because you want your friends to see it, you add and remove friends because you want or don't want to communicate with them. The side-effect is that your data is actively maintained and therefore relevant to share. A dedicated identity provider that doesn't have avenues for continuous interaction will quickly render your profile stale, except for the most resolute users who are willing to manually duplicate all their interactions from all other social networks.