Going dark seems like a reasonable policy for sites with a big audience and an outreach far beyound academe and intelligentsia. Wikipedia and Google are good examples of that. One day of Wikipedia being offline will have an impact on the outside world. MO going offline would be just useless (press won't write about it, politicians won't have troubles explaining things to their voters, etc.) and a case of preaching to the already converted (I don't think we have any SOPA/PIPA supporters in here). Note that arXiv isn't planning to strike either, and rightly so.
]]>It's a bit late to be saying this, and I'm not enormously well informed; but there it is.
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