That's a fine intention, but I sometimes like to compile code or use Chrome.
1. 12" is a weird size for me. I'd prefer 15" Pro edition.
2. One connector isn't enough, even with the adapter, I'd need more.
Casual Facebook users are not a target audience for anything, because almost everybody is a casual facebook user, including people who do 3D animation or need to compile huge codebases.
But I don't see that being realistic, yet, because (a) the new MacBook has less battery life than the 13" Air, (b) OS X is messier than iOS (system daemons love to run at 100% CPU once in a while), (c) as you said, almost everyone is a casual Facebook user and runs terrible crap like Flash. (iOS is at an advantage here again)
But yeah, it is a great device for people who only open their laptops occasionally and can recharge at night.