Students and teachers are the target audience. Salespeople are. Business people are. These people all have serious computing needs that are better met by an ultraportable device with long battery life than by anything that has more horsepower or more ports.
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.
The grandparent post was talking about all-day battery life, not about horsepower or ports.
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.