If you ignore the section which prohibits overt (and blatantly illegal) discrimination then the section which applies to this case is the "intention to discriminate" clause which requires Facebook to determine intent behind the targeting of the advertisements.

And, as we all know, nobody wants Facebook to be the Thought Police. Umm, well...most people don't want Facebook to be the Thought Police but that's besides the point.

Now (if HUD's arguments are valid) Facebook needs to determine if a landlord trying to run an efficient ad campaign or is making sure the "wrong kind of people" don't apply to be tenants. Either way the burden of proof is on Facebook since they are directly accountable for the thought processes of a third party simply by giving them the ability to discriminate against prospective tenants while (one would hope) not intending to discriminate against anyone themselves.

So, yes, I'm sticking by my original opinion that it isn't Facebook's responsibility to do HUD's job of rooting out the bad actors.