True, but the Fair Housing Act clearly covers advertisements for housing. 42 U.S. Code § 3604(c) ("it shall be unlawful . . . 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 discrimination.").

I think you can make a good argument either way for whether or not this prohibits the stuff Facebook did, but I'm leaning towards agreeing with HUD here.