Just watched the keynote. The social integration is very interesting. However, it now creates a situation for devices used by multiple people. In our home, iPads and iPods are multi-user. Grab one and go.
That means that one of several people could be on any device at any time. If I were to setup Facebook through Settings with my login information the integration will be broken for others. It also opens up some negatives. A seven year old can post to your FB account just as well as you can.
Then there's the scenario of handing your iDevice to someone else, say a house guest. They have full access to everything on your device.
It goes beyond that. Everyone's browsing history will now pollute the browsing history on your Mac. Everyone can edit documents and affect them on your Mac. Photos. Etc.
iOS6 is screaming for multi-user capabilities.