Moreover, BFT algorithms do not solve BGP in a decentralized, trustless network, consensus remains subjective. Bitcoin relies on proof-of-work to provide objective consensus.
Whitepaper: "We did not consider proof-of-work based protocols due to their poor performance and high energy (and environmental) costs"
There's no point in FB using a blockchain for their digital payments network, other than touting it as a 'cryptocurrency'. A centralized or federated database is all they need.
With supply cap of 21 million (17 million after counting lost/burned coins), the next block reward halving around the corner (from 12.5 BTC every 10 minutes, to 6.25 BTC), and assuming constant demand, price can only go but up. Halving ETA is late May 2020 https://1in21million.club/