They didn't make a mistake; Tess Holliday is obese and tells everyone that its okay. It's not okay, its unhealthy (I know, I'm the same way and know I need to get in shape; I'm not trying to by a hypocrite) it is hard to get back in shape and I don't blame her for being fat but I do blame her and her kind for perpetuating the "healthy at any weight" bullshit on the internet. She's not fat, she's obese; she's unhealthy and allowing her to tell people that it's okay is not okay. I know it's not okay for me to be obese, either, but I'm not telling people that it is.
Facebook, for once, did the right thing the first time; she shouldn't be allowed in an ad just like anorexic models shouldn't be either. Reversing their decision was the mistake.