If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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I don't think it's efficient. If you work for the amount of time you'd be spending giving blood and donate the money, you'd do much more good.
Possible, conditional on your income. Assuming we need blood, unless your work creates an efficient blood replacement alternative, someone must donate blood. Less Wrong's readership skews young with a lot of college students, who presumably have low income. If you're reading this, you are probably someone who has a comparative advantage in donating blood rather than money.