With this I agree. Though are we really ceding control over monetary policy to FB?

Hmm, that's a fair question. The way I see it, monetary policy is essentially managing the supply of money. The US creates and follows its monetary policy to manage its currency, the USD. This is effective at doing things like regulating inflation or easing recessions because we have one currency here: the USD.

But if Libra starts challenging the USD even just in the US, let alone in the rest of the world, suddenly there are two dials to turn: one for USD and one for Libra. As more value is held in Libra rather than USD, US monetary policy becomes less effective.

And this is just the money supply issue. Others have raised all kinds of issues about fiduciary duties, privacy, regulation, confiscation, security, and so on. While I wouldn't at all argue that the US approach to these issues is perfect (far from it), there are at least some laws and so on. Nothing would prevent Facebook from freezing your wallet, or freezing your account, or banning you from their platform, or tracking metadata on your purchases. I would also assume (perhaps incorrectly?) that law enforcement doesn't have to jump through the same hoops to get your payment/account information, so there's potentially more loss of rights there.

This is exactly what Facebook's vision for Libra is: an alternative currency to the USD shared internationally and controlled by their consortium. They say it will be pinned to the USD and other stable currencies, but there's nothing holding them to this (and in fact most cryptocurrencies play with this). I deeply believe allowing this to happen would be a grave mistake for all of us.

Is your objection primarily to FB controlling the cryptocurrency, or to the idea of (any) cryptocurrency threatening the USD?

I think when people aren't in control of the currency they use, as in can't change the supply of it, really bad things happen. Like, many more people would have died without the QE programs (recessions and depressions kill people). That's a major goal of CCs, and one of the main reasons I'm against them.