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'…
> 1. Is there a reason this is tied to the blockchain
For FB, you're 100% right. FB could have likely just used a relational DB in the short term. However, if you don't want to tie this to a single company (your second question), then a blockchain does make it easier for multiple entities to agree on the state of the db. I would argue that modern Proof of Stake protocols are starting to be competitive with Proof of Work in terms of security, in large part because unlike Proof of Work, they allow for both positive _and_ negative incentives for incentivizing behavior. For example, we've yet to see the types of double spend attacks on newer PoS chains (e.g. Cosmos) like the ones we saw on Ethereum Classic a few years ago.
> 2. Is there a reason this would be tied to a company
No, and in fact, this is what makes most permissionless blockchains so valuable. The problem is that most of them are not useable enough for what Facebook wants to accomplish. I say most because, in my view, Celo (https://celo.org and https://valoraapp.com) does. It's permissionless, programable, highly scalable, has a built in stablecoin and identity protocol, and most importantly, uses new zk-SNARK cryptography to let mobile apps sync trustlessly and near instantly. It also offsets 100% of its carbon emissions by buying carbon credits using block rewards.
> 3. Why a "new currency" ("stablecoin")?
I agree 100% and already the market is starting to tell us that USD pegged stablecoins are the most interesting of stablecoins.
> 4. I'm sure there are many issues with fraud/reversible transactions/KYC/anti-terrorism/anti-money-laundering that would need to be resolved somehow
Yes! This is an interesting read on the topic: https://www.coincenter.org/how-i-learned-to-stop-worrying-an...