If a Silicon Valley controlled currency becomes the preferred method of payment, then the companies controlling it get unchecked power over the economy.

Banned from Facebook? You need to find other ways to reach your audience and make money. Banned from Libra? Now you need to find another way to spend your money in a market that preferes Libra.

That's one possible bad ending, but maybe I'm a bit paranoid. Still, you'll also have to consider the dozens privacy issues that will pop up in the meantime, especially given Facebook doesn't currently have the best track record.