Granted. However, it is non-profit corporation, whose stated "business" purpose is spelled out in its Articles of Incorporation, and the Rules of the forum. Just because it is non-profit, and not expressly involved in commercial trade, it is still an entity whose overhead is substantial, and requires mass popularity to succeed at sustaining itself. Intentional harassments with the kind of posts alluded to are for the express purpose of damaging the corporate entity, through alienation and disaffection of its "membership".
That being said, I also unfortunately have to agree with you and doubt the likelihood of prevailing. And it certainly would not be cheap.
Like I said the first thing a judge is going to do is quiz FR about how it vets and controls its registrants, and then when he finds out it's much less than a LOT of other internet bulletin boards he's going to ask why you aren't doing much of that instead of bringing lawsuit.s