They say you lose the license if you make a claim "...that any right in any patent claim of Facebook is invalid or unenforceable." They also helpfully indicate what a claim means, and making an assertion counts.
Now why would they do this? My guess is that they wanted to have a legal tool that they could use to provide a chilling effect on any third parties who might volunteer information that is useful to someone filing a lawsuit against them. Which, if you are a lawyer, might seem like a good response to the demonstrated ability of groups like groklaw to crowdsource legal research. And if they were in such a lawsuit, that comment is exactly what Facebook would not want to see.
But there is no need to speculate. Their patent grant is quite clear, and the comment I pointed at is in violation.