It had just a 20% drop from week one to week two, which is phenomenal. It's now made $75 million in North America.
The reason is simple: it's timely and it's really well made. And it has crossed over into that "must see" zone where people keep talking about it and talking about it, and more people start going just to see what the fuss is about.
Also, African Americans go to the movies in really high numbers, too.