We need to go by the law here, which indicates an ad publisher is guilty in addition to the one who placed the ad. You may argue an opinion about the law, but it looks like Facebook is in violation of it.

I feel like FB could have covered their ass if they had a pop up menu for every rental ad that says "All ads must abide by the Fair Housing Act...click for more info. Check this box if you acknowledge your advertising practice abides by it"