In this case, just like Facebook.
Your responsibility, handed to you with the license, is to not do illegal things, even if people want to pay you money to do it. It's especially bad if you build an automated system that has options for people to do illegal things, since you are in charge of that. The FCC may or may not go after the people who bought the illegal-thing-service from you, but they will definitely go after you.
And the correct solution is to not have the options open in the first place. "Would you like to place an ad? Is it for real estate or housing in any way? OK, the following advertising options are open to you, but others have been turned off in order to comply with the law."