I don't think your metaphor is on point, either. I'm going to guess an ad in a majority-white med school is allowed (can I go on a limb and say most med schools are majority, white, anyways?).

But nothing is stopping a black person from seeing the ad.

In FB's case, it's like posting an ad at a med school (any, med school, regardless of demographics) and say...you can only view this add if you're white.