> and users can move freely between them.
The proprietary world has yet to invent a mechanism for that, and it's been a known problem for a long while (decades). Data "liberation" is challenging and, even if you don't think that is a problem, cross-realm authentication is all but nonexistent. They have little incentive to provide these things unless people demand them, and by and large people don't. (And before you bring up LiveJournal's OpenID protocol, I've two things to say: 1) it's not worthy of the trust placed in it because not all parties srongly authenticate each other, and 2) note that commercial OpenID providers do not, and fundamentally cannot by nature of the beast, make it easy to transition from an identity rooted at one to an identity rooted at another.)
The only truly distributed bring-your-identity-with-you schemes out there have come from the open, usually academic, world: PGP, SPKI/SDSI, E rights, the Petname system and protocol, and so on. Similarly, shared, secure-against-the-owner storage is not something social network companies have huge incentives to produce, but it exists in open research: TAHOE-LAFS exists and Diaspora has made vague promises to being similarly secure.