Updated 2/1/19: Apple has also banned Google's internal iOS apps too by revoking Google's certificate.
Citing a breach of agreement, Apple has revoked its distribution certificate from Facebook, abruptly powering down all apps using the intra-company iOS app distribution service. This does not mean that the Facebook app will stop working on your iPhone, but it does mean that Facebook as a company has lost its Apple Developer Enterprise Program certificate privileges for the time being.