What's the advantage of open information in the context of social applications? Excluding of course social networks built on mutual interest, like twitter or quora. With real-world friends, family, contacts, etc., the closed nature of facebook is what makes it attractive to me. And I think that's true for most users. It's probably why there's massive outrage every time they relax their privacy defaults.

Maybe data portability? But data on facebook is portable enough, and even if it weren't, what are you going to import it in? Some hypothetical future rival social network? I imagine for a majority of users, simply viewing a friend's profile to remember an email address, phone number, etc, is enough.