For HN, I believe that it won't make a huge difference to anyone if HN shuts down for one day, and as many people pointed out before, many HNers are well aware of the SOPA and going dark will not bring much further awareness.
For Google or Facebook, both 1 and 2 would prove worth acting on it. Reddit, maybe since they have a huge following & traffic. HN, clearly no point.
Rather than going dark, HNers would/should participate to the reddit initiative by voting up more SOPA related posts on the day Reddit goes dark, since HN is a contents hub. That would be way more productive.