Although facebook's coin will be the opposite of what cryptocurrency is meant to be (decentralized), we cannot deny the fact that this coin has a strong hype behind it because of the brand.
Regarding the threat to BCH, I think it's debatable because Facebook is the largest social media platform thus this coin will be greatly anticipated and adopted as time passes.
This is a real threat not just to BCH but to many online payment methods too. Anyways, I hope Facebook's coin will eventually bring more people to join the crypto field.