Allowing for such corrections could be one of the items that Facebook includes in those "terms of service" that no one ever reads. If people object to the idea that they may be having a correction to their libelous posting appear on their Facebook page, then they can opt not to use Facebook. (Consumer choice is great.)
Facebook and other Internet intermediaries often try to portray themselves as passive actors, like a bulletin board on the web. There are in fact many such passive bulletin board type sites on the web. The difference between these sites and Facebook, is that these sites don't make money from advertising or their participants' personal information. If Facebook wanted to go that route, then it could be exempted from responsibility for libelous material, just as a neighborhood bulletin board would not be held responsible for libelous material.
However, if Facebook wants to compete with print and broadcast outlets for advertising dollars, it should be held to the same rules as these outlets. There is no justification for special treatment for rich jerks from Harvard. There is no reason to waste any effort trying to convince Mark Zuckerberg what is good for democracy. We just have to tell him, and make him a pay a very big price if he is too lazy or dumb to "get" it.
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