The main function about cryptocurrrncy is decentralized but fb coin would be centralized I think,so how we tell it's for next Bitcoin rival.
I do not know if it would be centralized but it would be an extra addition to the crypto space and I believe it will spread the word faster than the existing process. Facebook coin would gain big value in future if it is introduced in the market because like BNB-backed by Binance, Facebook coin will be backed by Facebook and might have good demand.