The empirical evidence presented by OP's blog post is far heavier than your objections to it. You have some valid points, but so does he. All other things equal, he has cited scientific studies and you haven't.
I am biased, because my reading has predisposed me to agree with OP that multitasking is the antithesis of productivity, but that's just my .02.
Well, not really. Lets take objection #2 as an example. You wrote:
"""
2) Incorrect conclusions: "Students who browse Facebook and instant messaging while doing homework on average achieve lower grades in school".
"""
Which is a reasonable objection on the grounds that correlation does not imply causation. But keep that in mind: does not imply. The fact that something does not imply, does not mean it is necessarily false, or that the study was useless! In fact, the study gives us a lot more information than we had otherwise. So, it's better to draw an inference that's based on results, than none at all.
You go on to say:
"""
Really? Don't you think that students who browse FB instead of doing homework are probably bound to do worse in class not because of multitasking but because they are just not interested in learning?
"""
Which is a hypothesis completely untested! Perhaps you should do a study.
I am biased, because my reading has predisposed me to agree with OP that multitasking is the antithesis of productivity, but that's just my .02.