Earlier quoted context omitted.
42 U.S. Code § 3604. ...it shall be unlawful— (c) To make, print, or publish, or cause to be made, printed, or published any notice, statement, or advertisement, with respect to the sale or rental of a dwelling that indicates any preference, limitation, or discrimination based on race, color, religion, sex, handicap,familial status, or national origin, or an intention to make any such preference, limitation, or discr…
Yep. > or cause to be made... Key words here. Sure, the government should be going after individual advertisers too, but selective prosecution or not, Facebook is clearly in violation.
Facebook is just the hardware store in this sense. They provide tools. And you make the billboard. They did not cause this to be made.
edit: They still may be responsible for publishing it, which is illegal. I would think its a much better argument to go after them for that then the 'cause' to be made' argument.