For those outside the US who may be wondering why this is so significant:

https://en.m.wikipedia.org/wiki/Redlining

What exactly is wrong with excluding certain areas from seeing your ad? If I'm selling high end jewelry, for example, I don't want my ad shown to people in low income areas, because every click is a waste of money. Similarly, a discount store in a bad part of town may want to exclude wealthy neighborhoods, as people from those neighborhoods are unlikely to come to the store. That doesn't seem like something anyone should be upset about...it's common sense.

I guess it's a little more murky when it comes to housing, but how many people in downtown Watts are going to be legitimate potential leads to rent a house in Beverly Hills? I'm not sure that advertisers should be forced to pay for ads to be shown to people that cannot afford or would otherwise not be interested in what they are trying to sell.

Redlining does not mean simply drawing red polygons on the map. To be guilty of redlining, Facebook must have known the ethnic/racial/religious composition of the excluded areas and it must have known that the advertisers were using the tools for discrimination. Quite a tall order.