I'm just looking at the freshly-out OpenSocial API by Google, and a few others. While there's a lot of rationale behind how and why the Facebook platform works as we know it today, I understand such a system would be hard to use generally for a lot of social network providers, therefore the OpenSocial API is much simpler.
Looking at the API, it feels to me it perfectly fits some usual PEP-PIP-POP, (generally P.?P:) scenarios, and since some of the major XMPP server vendors contain a built-in webserver already, it shouldn't be hard to do as a module for them.
What would be the benefit of providing such API? Building systems on top of XMPP allows us to deploy services in the social web scene, which did not get jabber yet as much as it could be.