The answer is simple; they're not. According to Henry Blodget at alleyinsider.com:
This move seems another smart step toward a hybrid strategy: Allow app makers (and Facebook) to extend social-graph functionality to the web, gather more app users, and recruit more members--but retain full control over the social graph itself.This couldn't be any closer to the definition of "closed". Fortunately for us, there are alternatives. OpenID, while still somewhat nascent, sets the foundation for a truly open design.