I think students often don’t think about the public nature of the internet, and they also don’t always remember that professors are people too.

N– the site actually requires students to say which semester they took a course with the professor and which course it was (though, of course, this does not ensure they’d tell the truth). Like you, I would ignore it.

The update on Mano’s blog says that after she sent the message to her students, the offensive comment was deleted (presumably by its author) and she may well retract her decision not to write recommendations (which I think is a good idea). In effect, it looks as if it will go the way N suggests above.

I personally probably would have ignored this kind of thing – students are, by definition, learning, and don’t always do the wisest things: I’ve once been at the back of a crowded elevator while a clump of students from one of my classes was gabbing away about my course, not realising I was there… I think there’s a bit of an obligation not to react to this kind of thing, but to allow the students to be judged on their “public face” presented in class…

That said, if someone wants to bring home a lesson about the fact that these sorts of comments are inappropriate – or that, contrary to some student belief, the internet is really not a private space, there are other ways – maybe using the incident as a “teachable moment”? Maybe offering the opportunity for the student to come forward and apologise? Maybe threatening to withhold recommendations unless the student withdraws that comment from the website (an action they could still undertake anonymously)?