You've greatly misunderstood what facebook.stackoverflow.com is. It is not a separate site. All it is is a filter. Anything that appears on facebook.stackoverflow.com exists on stackoverflow.com proper. Take any question you see on facebook.stackoverflow.com and remove facebook from the url. You'll see the exact same question on SO proper.
A question entered on facebook.stackoverflow.com is visible to anyone on stackoverflow.com, not just the subsite.
So yes, if it is a valid programming question then it is perfectly fine on stackoverflow.com and flagging these as off-topic is inappropriate.