I think on a more general level people might post speculation about this political group, that professional group, some religious group, etc. and never once consider that groups are made of individuals. When you suggest any group might think/believe/do X then you are implicating individuals as well. When you indtroduce your specualtions into a public forum, especially one as vast as fb then there is a chance that people who knw or even are some of those individuals will see and take offense at the implications. For this reason when there is evidence disproving, as in OP's case, I don't think it has to be done in a PM. And I don't think the person posting should be hurt or suprised by the fallout. And the reality is that while this is one of the most recent and emotional examples, there ae several religious groups, political groups, professional groups, social groups, etc. who have had people "wonder" what they might be capable of. Introducing those suggestions are incredibly hurtful to individuals as well as groups.