Here are the words of “Chofetz Chaim | A Lesson a Day,” Day 14 (emphasis added):
All forms of lashon hora are prohibited even when no names are mentioned, if it will be possible for people to determine who is being discussed.
Well then, if people will not be able to determine who is being discussed, it sounds as though speaking negatively about a Jew is permitted. (This directly contradicts Little Froggie.) Note, however (same source):
If names are omitted but the story will reflect badly on an entire group of people, it may not be spoken.
Of course, the same question applies here – are the users of a certain website considered a specific group of Jews?
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Dash points out that someone else may know who the poster you’re talking about is. I assume this is quite rare.
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Little Froggie correctly points out that if your words will cause someone distress, there is a problem of ona’as d’varim even if there is no problem of lashon hora.
However, this does not apply to all cases, such as
when someone doesn’t use the CR any more.
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Showjoe answers yes to the original question, to which I say: Says which posek? I wouldn’t just assume this.
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Even assuming that a poster has the same din as a person in real life, to’eles must be considered. (I don’t know how to’eles would interact with ona’as d’varim here, where everyone can see everything.)