Why would Facebook have any interest in this? Public trust? Is the public that smart?

my guess is if facebook is working on its own blockchain currency, the algorithm describing the currency would be a bit different from bitcoin or ethereum. Contrary to what you read sometimes, those cryptocurrencies have properties that are not desirable for a real world currency ( aka inflationary and very poor number of transaction per seconds capacity).

If bitcoin and eth don't have a stable value in 2018, how does WhatsAppCoin sans Facebook guarantee have to stay stable?

I would imagine because actual commerce is built around it, rather than its valuation being mostly derived from speculative investment as a value hold.

Is anyone actually more skeptical in the long-term future of Facebook than of any altcoin?

(Remember that in order to prevent double-spend attacks, you need to ensure that nobody—and no potential mining pool—is sitting on more computing power than your network has. That's feasible for a single proof-of-work currency; it's technically impossble with even two unless one outcompetes and kills the other, and certainly practically impossible for a new one at this point. And if you're not using proof-of-work, what stability does the coin have once the corporate backer is gone?)

...or it could be they want to literally print money and have people use it in everyday commerce so they become a de facto Central Bank.

If it were merely an account one used to pay for things then they just get to take a small percentage in transaction fees but with a blockchain they can create a bunch of FaceCoins, sell them on the open market and if things go south with their cryptocurrency laugh all the way to the bank counting their billions.