Ok, so let's focus on the underlying idea, without the "Facebook is trying to rebrand their evil brand" cruft. Basically, I think the underlying idea is valuable - low-cost, fast, reliable, accessible money transfers - clearly something missing right now in this world, even for the "global rich" (citizens of first-world countries), let alone for the "unbanked". It could even scale even more to a money store but that'…
That's quite a high bar for rival payment methods to clear, but I think the main problem for corporate currencies like this will be rivalry from government issuers. Governments jealously guard the right to issue currency, as they rightly perceive it as one of their biggest holds over citizens. Any sufficiently powerful currency would attract their scrutiny.
I do think payments are ripe for disruption though - when money is sent over a free global network between pre-vetted partners (i.e. bank customers) there is no reason the transaction fees should be high, even internationally. I'm not clear how a blockchain based currency is going to help facilitate that, but would love to see a revolution in payments which made them simpler and not tied to archaic concepts like sort codes and account numbers, and reduced the fees charged to customers and merchants for payment networks or card issuers which don't do the hard KYC work (banks and payment processors do that).
The problems here are in verified identity tied to real people, not how to perform transactions quickly, or things like anonymous trust-free transactions which nobody actually wants.