Typically, porn-related ads are a sign of a desperate website.
Creating an advert that takes advantage of the Facebook API, and than buying screen time on Facebook does not involve human interaction of any kind on Facebook's part.
A flaw? Perhaps. But your thesis is not supported by this occurrence.
First, there is nothing wrong at all about advertising adult services to adults. While the majority of Americans are prudes, there are some that are not.
Second, you know as well as I do that as soon as Facebook became / becomes aware of this incident, they will remove it.
Sure, everyone knows that Facebook is derided and hated at Slashdot, but really, what else should they do? And anyway, Little Sophie from Sequim (pronounced "Skwim" by the way) will be set after her parents hire a lawyer extort a few hundred thousand from Facebook for "psychological damage and suffering".