First, I am not a litigator, so you would need one to evaluate if you have a claim, the likelihood of obtaining redress, and the cost. That said, the typical issues with defamation is not just that its something untrue, but a) what damage was suffered by reason of the statement (most but not all defamation claims require a showing of actual damage), and b) the cost to pursue. The non-legal answer is that I have found such postings to be frustrating but do now warrant taking any action. The likelihood of recovery, or obtaining an injunction for such comments would be difficult. Not to mention, people posting things like this are disgruntled and responding only emboldens them to do more. Moreover, ultimately the film speaks for itself. Not how you got to the finished film or what issues, true or false, happened a long the way. Distributors don’t care about the back story. So, all in all, I say move on.