There are only a few cases in which the stock vote reasons should be changed. The Vote to Close dialog shouldn't be too large; the existing reasons cover the vast majority of possibilities already and it's not a crime to mis-categorize a question that ought to be closed.
However, we already have site-specific off-topic close reasons. The Facebook sub-site
facebook.stackoverflow.com is an anomaly, but could (in theory, I'm not sure how it works technically) get a customization of the text which currently read:
$SITE_NAME are expected to generally relate to
$SITE_TOPIC, within the scope defined in the faq.
Questions on Stack Overflow are expected to generally relate to programming or software development in some way, within the scope defined in the faq.
Questions on Gaming - Stack Exchange are expected to generally relate to gaming, within the scope defined in the faq.
Questions on Electrical Engineering - Stack Exchange are expected to generally relate to
electronics design, within the scope defined in the faq.
Sure, it's not a lot of customization, but there's room for some creativity, like the bolded text below:
Questions on Facebook - Stack Overflow are expected to generally relate to technical questions about developing Facebook applications. This is not Facebook Support. Call Facebook[phone number needed] or visit http://www.facebook.com/groups/fbdevelopers/ for administrative help with Facebook's backend database. Questions on this site should be technical, not administrative, within the scope defined in the faq*.
The right action to take is not to create a sixth close reason for this specific case (in which no less than 10 comments were expended to clarify the issue), it's to update the existing off-topic close reason.
* - Note