"My point is that I don't understand why Facebook would need a Cryptocurrency to allow payments"
Let me try to explain. You trust your bank and PayPal and it's perfectly fine. But only to the point things are going ok in general.
In some, say, areas, the situation is possible when you see a message on the doors and ATMs of your previously trusted for decades banks: "Sorry, no check out today". And it can long for days or even months.
And your previously trusted online e-money processor can freeze your funds for whatever reason - "fraud suspect, suspicious activities, server offline".
It is hard to imagine for citizens of some countries, but in my opinion, this is only matter of time when it happens.
Cryptocurrency and blockchains in general are indeed highly inefficient databases and a tiny Postgres server outperform them all in Transactions Per Second, that's true.
But Postgres database running on a centralized service won't give you a feeling of independence and trust the biggest cryptocurrencies can provide (sure, they are volatile in price and you need to manage your private keys etc, but still).