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.
The most interesting fact, is that Facebook, will attract more people in the cypto field. So if new users will buy Facebook coin, they will know definitley also other major coins and blockchain bringing advantages to the world sector.