I think the word 'claim' is used a little ambiguously, but given how it is used in the rest of the patent declaration, I don't think the intent of Facebook is to stop you from saying anything about infringement, rather it is to stop you from filing a claim in court or with the patent office.

Don't worry about intent, worry about what they actually say. They have smart lawyers, and wouldn't say it if they didn't mean it.

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.