It wasn’t in the App Store. It was sideloaded from a Facebook website using a profile signed by Facebook’s enterprise developer certificate. Screenshots in various news articles confirm this.
It violated the Enterprise Developer Certificate Terms, so Apple have revoked the Developer certificate.
The rules state that Enterprise apps must only be in the hands of users who are under the supervision of the company which operates the Enteprise certificate. It's likely that Apple doesn't consider the people Facebook have co-opted into this program to qualify
